Peking — China is serene in face of its glaring military inferiority to the Soviet Union. This contrasts sharply with the urgency displayed by the Reagan White House over Washington's arms race with Moscow.
Nowhere is the Sino-Soviet disparity more apparent than in the air, where Chinese versions of the MIG-19 and MIG-21 are perhaps 20 years behind the latest Soviet fighter planes.
Yet when Defense Secretary Caspar Weinberger visited Peking at the end of last month to preach Sino-American defense cooperation against the Soviet threat , he found his hosts, military as well as political, unanimous on one point: Modernization of China's defense forces can be achieved only within the framework of overall economic modernization.
This means that agriculture, light and heavy industry, and science and technology all have priority over military modernization. In Xian, Mr. Weinberger was shown a 40,000-employee factory that produces engines for the Hong-5 and Hong-6 bombers. These are China's only bombers, the equivalent of Moscow's IL-28 and TU-16, respectively.
China started making these planes in the late 1950s under Soviet guidance. When Moscow abruptly withdrew its experts with all their technological information in 1960, the Chinese had to continue as best they could. The Hong-5 and Hong-6 are carefully made and include some improvements over the Soviet models. But they still represent a technology that is well over two decades old.
The Xian factory is well known in the West because it imported Rolls-Royce technology to make the Spey jet engine, a commercial engine that could have military applications.
The Chinese are embarrassed about the Spey engine. Apparently they did not succeed in designing an appropriate airframe for the engine. According to Chen Zheng, chief engineer of the factory, the techonology was imported in 1977 and ''about 10'' engines were actually built.
In numbers, China's Air Force is the third largest in the world, outranked only by those of the United States and the Soviet Union. It has 490,000 servicemen including 220,000 air defense personnel, according to the latest issue of ''The Military Balance,'' published annually by the London-based International Institute for Strategic Studies.
It has some 5,300 combat aircraft, the mainstay being some 3,000 J-6s, the equivalent of the Soviet MIG-19. The J-6s have been improved over the original MIG-19, and late versions feature a needle-nosed radar. The Air Force also has about 300 J-7s, the equivalent of the MIG-21. Its latest fighter, known by various names in the West but most commonly as the F-8, corresponds to the Soviet MIG-23 and is a variable-wing fighter.
Some Western sources believe there may be about 30 of these planes in service. But others say the plane has been beset by problems and doubt whether it is in use except on an experimental basis.
Chinese defense experts obviously are not satisfied with this. But they show none of the near desperation that their counterparts in the Pentagon might feel if facing a similar gap vis-a-vis the Soviets.
''Yes, we will spend more on fighter planes and other defense equipment, when our economy is a bit richer,'' said one Chinese source. ''But meanwhile we have our missile and nuclear weapons, and we have a huge country with a billion people in it.''
''Afghanistan is far smaller and less well-armed than China, yet look how the Soviets are stuck there. Vietnam is smaller and less powerful than China also, but look what happened to you Americans there.''
'' Do you think it is really credible that Andropov would dare to attack us?''
Under technology-minded Defense Minister Zhang Aiping, China's defense establishment, including the Air Force, is improving its equipment wherever it can.
But, as of today, it is concentrating principally on training a new generation of pilots and staff officers who are thoroughly grounded in science and technology.
''Our nation's material and technical foundation is still relatively weak,'' said armed forces Chief of Staff Yang Dezhi at the National People's Congress here in June. ''The modernization of our national defense can be achieved only step by step, stage by stage. . . . The part must subordinate itself to the whole. We must try our best to build the armed forces with the entire situation in the country as guidance.''