For 18 months, Joe St. Clair has been one of a handful of Americans in China allowed to live outside this capital city in an area so remote and politically sensitive the Chinese have not let U.S. diplomats stationed here visit it.

In a little mountain valley 1,100 miles southwest of Peking, St. Clair's company, Pullman Kellogg of Houston, has been building a factory for making fertilizer ammonia. In the process, it has created a small oasis of pork chops, air conditioning, plastic Christmas trees and Mary Tyler Moore in the middle of the ancient Chinese countryside.

St. Clair tells of close ties developing between Americans families and local Chinese, and of American job supervisors, bewildered by the quirks of Chinese industrial democracy. Working in an area not far from a center of bitter political strife, the Americans sometimes found guards suddenly swarming over a nearby rail depot, but no serious trouble ever came to their rural job site.

St. Clair said he felt hemmed in, since some outlying areas were off limits and there was nothing to do there anyway. "We had lots of things to keep us busy. But I missed being able to go out on the town at night," said St. Clair, a 28-year-old materials manager who said he still enjoyed his stay in deepest China.

During the early 20th century, when the last imperial dynasty crumbled and everyone from Christian missionaries to cigarette merchants poured in, perhaps 10,000 Americans lived in this country. Then the Japanese invasion and the Communist liberation reduced the number of Americans in China to almost none.

Now, after five years of renewed Sino-American relations, China has become the home of 187 U.S. citizens, a mixed bag of diplomats, oil derrick salesman and construction men like St. Clair. Their work and lives here provide a clue to the future of U.S. contact with the nation that Americans have always seen as the world's great untapped market.

Since Pullman Kelloge signed a $220 million contract in 1974 to build fertilizer plants at eight remote sites for the Chinese, the company has rivalled the U.S. Liaison Office in Peking as the largest single American group in China. Pullman Kellogg now has about 45 American employees and dependents in the country, with four of the plants already completed. The company once had as many as 120 Americans living here, far more than the 66 American adults and children attached in some way to the recently expanded U.S. Liaison Office here.

Mindful of Chinese sensitivity about foreign assistance. Pullman Kellogg employees have usually declined interviews about their China experiences even after returning to the United States. But with China's newly relaxed attitude toward importing technology, St. Clair and John Churila, manager of the company's Peking office where he has been reassigned, agreed to be interviewed.

"I really didn't know what to expect when I came here," said St. Clair, who had never been out of North America. The peasants and workers of northeast Yunnan Province, the location of what the company cells site No. 5, had seen few if any foreigners, and they had to get used to 18 Americans and another six or seven Europeans building a fertilizer urea plant next door.

"The kids of the men with families would play with the Chinese kids, learning some Chinese games and teaching some American ones. Some of them picked up Chinese a lot faster than I did," St. Clair said. American mothers taught their children through correspondence courses.

The company had built air-conditioned quarters for its staff, including dining hall and recreation room complete with a videotape-playing television set. "We'd get tapes of Mary Tyler Moore, Rhoda, M.A.S.H., the Movie of the Week, We's play them and then send them on to another site, and have to wait for the next batch to arrive," he said.

To fill the time, the American developed an age-old Chinese knack for turning eating into a form of recreation. "We had a western menu and a Chinese menu. Most people stuck with pork chops, steak and other western food, but I liked to try the Chinese food. I tried everything. Then people would ask me how I liked it and try it themselves. After a while we organized a regular Saturday night banquet where everyone would get another and try the cook's favorite dishes," St. Clair said.

On the job site, frustration arose occassionally as the Americans tried to direct work over a cultural and political chasm. "They have to have a meeting to decide just about everything," said St. Clair of the Chinese workers on the site.

"The operating supervisor would explain how a job was done, and some little fellow down over there would say, 'Oh, I don't think it should be done that way.' Then they'd go have a meeting about it," Churila said.

"That was all right during construction," Churila said, "but what if that happened when something went wrong while the plant was operating?

"Fortunately, since the fall of the 'Gang of Four' we've noticed that this situation has improved greatly," he said. Until last October, China was plagued by a power strugglt between a group of pragmatic bureaucrats in the Communist Party and army and some more idealistic Peking leaders who insisted the thought of Mao Tse-tung, including the right of workers to challenge the way their plants operated, be observed to the letter.

Those who insisted on Maoist dogma, including Mao's wife, Chiang Ching, and what Peking calls her "Gang of Four," were purged shortly after Mao's death.

The new administration of Chairman Hua Kuo-feng has emphasized obedience to duly authorized supervisors in factories, offices and schools. In the meantime, many Americans have learned to follow a simple rule when they aren't communicating.

"Just step back and let the Chinese go their own way," Churila said. "They'll get to the same place."

All the important American holidays were observed. For Christmas the company sent in green plastic trees which entranced the Chinese. "They would think of all kinds of excuses to come in and look at the tree," St. Clair said. The Fourth of July was much noisier than in much of the United States, for the Chinese don't restrict the use of firecrackers, one of their most prized inventions.

The Americans were equally fascinated by Chinese rituals. At spring festival, the Communist name for the Chinese New Year, St. Clair and several others were invited to the provincial capital of Kunming where they were treated to a feast that still lights up the American's eyes. When Mao died, Churila was in a rural Hopei town of about 80,000 and watched as peasants in rough clothing poured into the town on foot for days to pay their last respects at a makeshift memorial hall.

Some of the Americans here, particularly some Pullman Kellogg staff and some diplomats at the U.S. Liaison Office, say they like the solitude one finds even in Peking. With no American television, newspapers or night life, there is time to read and reflect, to rediscover family life and conversation.

But St. Clair and others say they need some kind of safety valve. Diplomats here try to use their imaginations when throwing parties. St. Clair liked the Pullman Kellogg formula for mental health, one week's vacation out of the country for every six months of work. He usually went to Hong Kong, a virtual cornucopia of the fruits of Western life.

If the number of Americans living in this country increases as relations improve, the need for Hong Kong will become all the greater, particularly for the young. At the famous Lo Wu bridge from China to Hong Kong, the People's Liberation Army guards watched quizzically one recent morning as the 13-year-old son of an American resident of China crossed the border. Waving his arms, "I'm gonna buy the eagles," he shouted, "I'm gonna buy the stones. I'm gonna buy everything."