Zhang Wenjin was 15 when he first discovered Marxism. Born in Peking, the eldest of four children, he was sent by his family to the German school in Shanghai. Later his father, a businessman, agreed he should study in Germany.

It was 1930 and Zhang was in Berlin when Chou En-lai passed through on his way to Moscow to attend the Communist International Executive Committee session. He was traveling in disguise because Chiang Kai-shek had put a high price on his head as one of the Chinese Communist Party's key leaders. Zhang remembered Chou--at this, their first meeting--as inspirational, serious and also very self-disciplined.

"My father wanted me to be independent as early as possible," said Zhang, "but he didn't want me to be a Communist--he never dreamed of that. He regretted it at first, but, of course, later he was happy because then he saw that the Communists really were serving the people."

Now, the 68-year-old Zhang is not only a Communist, but the top diplomat in the United States for the People's Republic of China. As its newly appointed ambassador, Zhang will present his credentials to President Reagan today at the White House.

The timing leaves something to be desired.

Normally a routine ceremony exuding bilateral friendship and good will, the one featuring Zhang will take place in an atmosphere of chilled U.S.-China relations.

Monday, in the latest strain between the two countries and after a nine-month interlude, the Justice Department announced that China's top female tennis player, who defected in California last summer, had been granted political asylum. The U.S. action has provoked angry reaction from Peking, where officials charged that Hu Na, 19, had been coerced to defect by the "collusion" of Americans and Taiwanese agents.

At the Chinese Embassy the day after the announcement of asylum, in controlled, conversational and emphatic English, Zhang repeated that position. He called the U.S. action "connived . . . deliberate . . . a provocation" and indicated that it would put "a shadow" on future visits to this country by other Chinese, including scholars, sports figures and musicians.

Zhang agreed to the interview, the first he has given since arriving here, several days before the U.S. Immigration and Naturalization Service made its decision. The ground rules then were that there be "no specifics" on political matters during the interview, but those seemed to have been forgotten. He sat at one end of a beige upholstered couch whose back, like those on a matching couch and chairs, was covered with the white lace antimacassars the Chinese even put in train compartments.

A slender, aristocratic-looking man in a western business suit and tie, Zhang seemed relaxed as he talked about his life, China's political policies and what he hoped to accomplish in his new post. He laughed at times, joking about Washington's social life, which he had already sampled at several diplomatic receptions.

"I enjoy myself," he said. "That's part of my job."

Sitting at Zhang's elbow was Yang Tiechi, his interpreter and assistant, who lives with the ambassador and his wife, Ying, in the ambassador's residence a few blocks away. Across the room was First Secretary Yu Zhizhong. Without the aid of any visible tape recorder, both took notes of Zhang's remarks, and occasionally, when asked, supplied a word in English.

Sketching out how his government attempted to forestall the U.S. decision, Zhang said that when Secretary of State George Shultz visited Peking in February, the Chinese reasserted their position. Shultz's response, said Zhang, was that it was a Department of Justice matter and that he didn't know what the outcome would be.

"He says the Americans have their own laws," said Zhang. "Well, there was no satisfactory answer."

Zhang got an earlier glimpse of American ways in 1973 when he went to Canada as ambassador, staying there until 1976. He ate hamburgers and Kentucky Fried Chicken and watched the United States as a spectator sport.

"At that time we had no diplomatic relations, so I could only look at the United States through television. I watched the election campaigns over NBC, ABC and CBS--that's all my knowledge about it," he said.

He said he used to ask the Canadians what the difference was between them and Americans and they would say, " 'Well, we don't know. We know that we are not Americans, that's one difference.' "

In Ottawa, Zhang also followed the social career of then-Iranian Ambassador Ardeshir Zahedi.

"He was very famous. I heard much about him," said Zhang.

Did Zhang plan to become another Zahedi in a town that is always looking for a new host to lionize?

"I don't have the money. My country doesn't give me the budget," he laughed. "The Iranians are so rich--the shah was, particularly the shah."

Zhang said he plans to entertain, though, "I don't know how much," and he asked for some tips on how to go about it. He said he and his wife gave dinners and receptions in Canada, and that they expect to do that here. "On National Day, definitely, and also on my arrival--an introduction."

He has already had a taste of being a host in Washington, he said, when he was kept busy entertaining members of Congress and senators who were going to China during the Easter recess. He pointed out there was no special entertainment budget for the Chinese ambassador.

Neither is there a travel budget to bring Zhang's three sons and one daughter to the United States for a visit. Two of them are married and have children. "They have their own work and no one pays for their traveling expenses," he said.

When Zhang first met Chou En-lai "he was very well known in China," said Zhang, one of a half-dozen Chinese youths who gathered that day in Berlin to hear Chou. "He spoke to us about two hours--free talk--briefing us on the latest revolutionary situation in China. I was very much impressed."

Zhang didn't see Chou again until some years later. In the meantime, Zhang had studied mechanical engineering at Peking's Qing Hua University--"not Peking University," said Zhang, hoping to clear up that oft-repeated error. He never made the Long March, but he said he was active in the student movement. Then, around 1944, he met Chou again.

"Chou En-lai stayed most of the war in Chongqing as the representative of the Communist Party. I was wandering about doing all kinds of business and as an underground worker," said Zhang. His father and Chou had gone to the university together, he later learned, and Chou often spoke of it.

Around 1944, proficient in English as well as Chinese and German, Zhang was recruited into the communist party's liaison office. Instead of engineering, he worked with foreigners stationed in Chongqing--embassy staffs, journalists, missionaries and a number of groups doing relief work.

After the Japanese were defeated and Gen. George C. Marshall was sent by the Americans to mediate between Chiang Kai-Shek and the communists, Chou En-lai took Zhang as his interpreter.

"Chiang Kai-shek wanted to start a civil war and the Communist Party wanted to have peace and a coalition government. I think the Americans also wanted to prevent a civil war," said Zhang. "But the word was that the American government wanted to support Chiang Kai-shek . . . so Chiang took advantage of the Americans' support to launch his civil war. Marshall's mission failed and he returned home."

Zhang remembered that Marshall always offered them drinks and "I asked for tea, so he gave me iced tea. I enjoyed that. It was very nice in summer because it was very hot."

At their final meeting, Zhang remembered, Chou En-lai told Marshall how disappointed he was with the American policy and he regretted that the American government was unable to see how it had given Chiang too much military support.

"They actually encouraged him to start the war," said Zhang. "It was bound to be a failure."

Later, when the communists had driven Chiang off the mainland and China became the People's Republic, Zhang began to work within the new government's foreign ministry. By 1956 he headed its Asian affairs department. In 1966 he went off to Pakistan as ambassador, only to be called home a year later when the Cultural Revolution began. Sent to the foreign ministry's cadre school, called the May 7 School, in Jiangxi Province, he learned to plant rice.

"I've been eating rice all my life," he said, "but I had never planted rice myself."

For six months he lived in the same room and worked side by side with the "rank and file, the cadre and the peasants." He said his physical strength increased as well as his knowledge of farming. Some in poor health suffered, he said, "but for myself and for most, I think, it's a good experience so long as it's not too long."

After he left, he learned that the farmer who had taught him, upon learning that he was an ambassador, had called him "not a bad guy," just "not a good farmer."

Recently, when China's Gang of Four went on trial for its leadership in the Cultural Revolution, there was "real rejoicing" among the Chinese, particularly in Peking and Shanghai, Zhang said. "People have only hatred for the Gang of Four . . . They got justice."

In 1971 Zhang was director of the foreign ministry's West European, American and Australian Affairs Department, and by 1972 he was also assistant to the foreign minister. That's when he met Henry Kissinger. Zhang was working as a member of the PRC team on the Shanghai Communique', the first step toward normalization of relations with the United States.

"It was very strenuous and very hard work. Most of it Chou En-lai did himself, actually. He used to work through the night," said Zhang, "and I remember during Kissinger's second visit to China, I think in late October, it took about a whole week mainly for the preparation of Nixon's visit."

Zhang remembered that Chou and Kissinger, who had a "very good friendship" by the time they parted, exchanged views on all kinds of issues as they worked through the Shanghai Communique'.

"Kissinger said he had never before participated in such a communique', where each side stayed at his own position but also made clear where the differences were and where they might have common points," said Zhang.

Giving Nixon "high credit" for bringing the two countries together, Zhang said it was Nixon "who made the decision. Kissinger was ingenious in carrying out that very difficult mission--almost impossible--but he succeeded."

In 1978 Zhang participated in negotiations that led to a claims asset agreement between the United States and the People's Republic and to the opening of consulates in both countries. He was also a vice foreign minister, holding the post until coming to Washington a few weeks ago.

It is his second time in the United States; the first was in 1979 when he accompanied Deputy Premier Deng Xiaoping after the normalization of relations.

Now that he has come back to stay for a while, he sees his job this way: "I hope that I can make the American government and the American people have a better understanding of China . . .

"But we also do not overlook the difficulties and the obstacles that are still there . . . The main difficulty is on the American side. You know, this Taiwan relations act passed by the Congress is incompatible with the principles that we, of course, are agreed to . . . We strongly object to the American sales of arms to Taiwan, which we consider an infringement of Chinese sovereignty and interference in Chinese domestic policy."

Despite such differences, he said he intends to have as much contact with Americans as possible.

"I think the best way to understand a country is to see and to meet the people."