American military planners, in their first meaningfull but brief look at China's Army, Navy and Air Force, have come away with an impression of an immense, rusty and old-fashioned military trying to polish and modernize itself into a new strategic potency.

"They'll tell you they're backward, that their equipment is antiquated," said one American expert surveying Chinese missile destroyer during the visit last week of U.S. Defense Secretary Harold Brown. "Yet they take pride in this ship; it's well maintained, and this hydraulic steering system is advanced."

The Chinese have the advantage of enormous manpower and the sort of eagerness among youth for the rigors of military life that is only found in poor peasant societies such as this one. "We still have far more volunteers than we can enlist," one Chinese officer said.

In its brief skirmish with Vietnamese troops a year ago, those overwhelming numbers, peasant courage and the well-maintained, if backward, equipment in Chinese storehouses performed well and brought a limited victory after initial confusion and mishaps. Foreign military strategists, however, suspect that the basic Chinese strategy against its most likely major war adversary, the Soviet Union, relies on material as outmoded as the T59 tanks the Chinese showed Brown during his visit.

The principal avowed Chinese military strategy remains the late chairman Mao Tse-tung's idea of "people's war," allowing a well-equipped invading army to plunge deep into Chinese territory, then sniping at its forces with militia and small unit engagements until it wearies of the struggle and withdraws. The Chinese extol this approach as a reaffirmation of the close ties among the Army, the Communist Party and the vast peasant masses. Actually, it is making a virtue out of a necessity. Peking is short of modern tanks, antitank weapons and aircraft that would make a full-scale response to a Soviet invasion possible.

U.S. military analysts on the Brown trip question whether the Soviets really would rise to the bait and launch a deep invasion intent on winning much Chinese territory, as the Japanese did in the 1930s when Mao evolved his military theory. If Moscow ever had any intention of attacking China at all, it is doubtful that the Soviets would want to fight on Chinese terms.

It would be better from the Soviet view, American analysts say, for Moscow to use its overwhelming air superiority for bombing raids on principal Chinese industrial cities in the three northeastern provinces once known as Manchruria. Punitive raids on Chinese border military camps also might be launched, then pulled back, or parts of the highly vulnerable border areas might be held for bargaining purposes in Xinjiang, in the northwest, or Heilongjiang, in the northeast. A small Soviet raiding party managed to cross the Amur River in Heilongjiang in mid-1978 for a few hours without apparently even being challenged by Chinese military units.

There appears to be a muted debate within the Chinese Army over how much to reply on "people's war," but most analysts think the discussion has been put off because there really is not enough money in the treasury to modernize the Army even if desirable. Instead, the Chinese are trying out small improvements and better discipline and management of the equipment and personnel -- estimated at about 4 million people in uniform -- that they have. They are shopping actively for European-made antitank and antiaircraft missiles. The British-made Harrier jump-jet, which could counter Soviet tanks and operate more easily along a northern border that lacks airstrips, has been high on the Chinese shopping list.

Peking seems intent on getting the best bargains, favorable credit terms and no obligations to buy more than a few pieces of equipment to start and does not seem troubled by the fact that this stubbornness is delaying the actual purchase date.

The one thoroughly modern arrow in the Chinese quiver is its nuclear strike force, although here it still relies on liquid-fueled rockets that cannot be fired quickly and on about 80 TU16 bombers with a radius of 2,000 miles. Peking is estimated to have about 30 to 40 medium-range ballistic missiles with a range of 600 to 700 miles and 50 to 70 intermediate-range missiles, with a range of about 1,500 to 1,700 miles, capable of reaching most Soviet cities east of the Urals.

Brown said during his visit that he "was certain" the Chinese are developing a 3,000- to 5,000-mile range intercontinental missile that could reach Moscow.

A Shanghai official newspaper published an article this week with photographs of future Chinese astronauts undergoing training at an undisclosed location. Another Shanghai newspaper published a photograph of a trainee's dog that it said had been rocketed into space and returned safely.

Aside from the light it threw on Chinese ambitions to someday put people in space, the article also served to display Chinese confidence that the rockets they were developing will be powerful enough to boost very large loads into orbit -- or reach enemy cities.

The Chinese have about 300 nuclear warheads, both atomic and hydrogen, and although the delivery systems are far behind the rapid-fire, solid-fuel missiles in the U.S. and Soviet arsenals, they still are engough to provide a credible nuclear threat.

U.S. military experts warned often during their visit here against underestimating the fighting ability of the Chinese forces, even if they did not possess the most up-to-date equipment.

"We don't make diesel submarines any more, haven't made them in years. All ours now are nuclear," one American officer traveling with Brown said. "But our crews would be just as concerned about a torpedo coming at them from a diesel sub as they would from a nuclear one, even if the nuclear sub doesn't have to surface as often."

At the 38th Fighter Division, a major stop on Brown's tour, the defense secretary remarked that the instruments in the F7 fighter he was shown showed substantial improvements by the Chinese over the original Mig 21 Soviet model. An American pilot there complimented the skills of the Chinese pilots who demonstrated maneuvers in the F6, their version of the Mig19.

The americans noted that Chinese pilots get on the average only 10 hours of air time each month a half to two-thirds what U.S. Air Force pilots receive, but they said that might not make a significant difference in their ability to meet combat objectives.

All the Chinese military officers visited here told Brown that they could not modernize their forces significantly unless the entire economy, especially the metals and electronic industries, were modernized. This will take time, and in the interim the Chinese do not seem to be weighing their budget heavily in favor of military expenditures.

The Chinese military budget of $12.9 billion, a figure first released last summer, does not include substantial expenses such as pay and allowances, but analysts figure military expenditures to be about 10 percent of China's gross national product.