Secretary of Defense Caspar W. Weinberger arrived in Asia today hoping to forge a new military relationship with the People's Republic of China.

Weinberger, who is to reach Peking Sunday after a one-day stay in Tokyo, said he is prepared to "discuss specifics" of weapon systems if China wants to buy U.S. arms. With U.S.-Soviet relations near an all-time low, the Reagan administration hopes to play on China's fears of Soviet expansionism and lead the Chinese gingerly toward closer ties with the United States.

"What we want to do is respond to their needs and their requests," Weinberger said shortly before arriving here. "As two great powers, we share a lot of strategic and economic interests."

Pentagon officials cautioned against expecting dramatic agreements on arms sales or military cooperation, however, and Weinberger assured Japanese officials today that the United States has "modest goals" in pursuing the Chinese relationship.

The two nations are only now emerging from two chilly years, in large part because of U.S. arms sales to Taiwan, and China increasingly has declared itself independent of both superpowers.

In addition, some Defense Department officials remain wary of giving China militarily useful technology, although administration policy calls for such trade. A new list of computers and other sophisticated technology to be made available to China was to have been published before Weinberger's trip, but it was held back for a time while U.S. officials sought assurances that the technology will not get to the Soviet Union.

U.S. officials also said they must take care not to alarm Japan or allies in Southeast Asia by arming China too enthusiastically. China, for its part, has made clear that military improvements must take second place to industrial modernization.

Nonetheless, as he traveled west for his first trip to China, Weinberger expressed almost unrestrained enthusiasm for helping that nation build its defenses. He suggested that the recent Soviet downing of a South Korean airliner and what he called a massive Soviet military buildup in Asia underlined the common interests of the United States and the three countries he is to visit during the next 10 days, Japan, China and Pakistan.

"We have the same basic interest in having them both be strong economically and strong militarily," Weinberger said, referring to China and Japan. "We do think the whole ability to deter war and maintain peace will be greatly increased if we can strengthen China in ways they want to be strengthened."

For U.S. strategists, the concentration of Soviet forces along the 6,000-mile Soviet-China border means that many fewer soldiers the United States and its allies must counter. But Pentagon officials also say Chinese troops along the border are no match for Soviet tanks, nuclear missiles and sophisticated military equipment.

As a result, Weinberger said China is interested in buying defensive weapons like antiaircraft missiles and early warning radar systems. When China and the United States first discussed arms sales more than two years ago, the Chinese are said to have expressed interest in Hawk antiaircraft missiles, Tow antitank missiles, improved metallurgical techniques for tank armor and other defensive aids.

Those discussions broke down because of the Taiwan issue, which essentially froze U.S.-China relations until Secretary of State George P. Shultz visited Peking in February. Now U.S. officials believe that China will buy only a few of any particular weapon, seeking to study the arms and arrange to produce them in China.

"If they want to discuss specifics, we're certainly prepared to do so," Weinberger said, "but I don't have any order books with me."

Above all, U.S. officials said they will not beseech the Chinese to buy weapons, though they would like to establish such a pipeline. Officials speak of a "new realism" in relations that calls for recognition of common interests, but no grand strategic alliances and no U.S. supplication.

U.S. officials said that tough-minded, no-bearing-gifts attitude characterized Shultz's visit in February and won the respect of Chinese officials.

The United States should not hesitate to express its dismay, they said, when China lumps the United States and the Soviet Union in the same "hegemonist" category or abstains, as happened recently, on a United Nations Security Council resolution condeming the downing of the Korean jet.

In a news conference shortly before he left for Peking, Weinberger said the airliner incident "revealed the paranoia of a corrupt governmental system" and urged Japan to boost its defense efforts against Soviet aggression.

U.S. officials would be pleased if Chinese officials concluded Weinberger's visit by saying they believe that Soviet "hegemonism"--particularly along their border, in Vietnam and Cambodia and in Afghanistan--represents the greater threat. But Weinberger carefully avoided any appearance of pushing the Chinese toward such a position.

The Chinese are expected to be particularly interested in Weinberger's comments on technology transfer, since he is one of the administrations's leading "hard-liners" on the issue. Commerce Secretary Malcolm Baldrige announced when he visited China in the spring that it could buy technological products with possible military applications, but how that policy will be implemented remains in question.

The list of approvable technology has been drafted, but items on it could be vetoed on a case-by-case basis. Pentagon officials say that will not be a problem, but they want a clear commitment from the Chinese that nothing sensitive will be passed on.

Any sales to China would have to be approved also by COCOM, an alliance of NATO countries and Japan charged with preventing the spread of western technology to communist countries. COCOM's cumbersome bureaucracy does not distinguish among communist nations, and Pentagon hard-liners said they worry that exceptions for China might offer loopholes to European countries that want to do more business with Soviet bloc countries.