China unveiled today its first income tax, a levy so sublimed only 21 out of China's billion or so population are going to have to pay it.

Announced at the National People's Congress as a way to "safeguard China's economic interests," the unique new tax seemed likely to touch only the pockets of a few hundred foreign journalists and businessmen here, at rates ranging up to 45 percent.

Congress delegates audibly tittered throughout the Great Hall of the People auditorium when it became clear that few, if any, of them asked to aprove the tax would have to pay it.

Even the salries of China's top leaders do not quite reach the $6,575 annual exemption allowed under the new law. Only 21 Chinese performers, artists and writers, particularly those getting royalties, earn more than that each year, congress officials said.

In a new spurt of China's proposals coming out of China's parliment, Congress Standing Committee Vice Chairman Peng Zhen who unveiled a 33 percent tax on Chinese joint ventures with foreign companies and new laws making it harder to marry and easier to divorce.

Congress official Gu Ming said the proposed income tax would range from 5 to 45 percent on salaries and wages and a flat 20 percent on royalties, interest and other income. Days before the congress opened, foreigners living here, except for diplomats who are exempt, received notices of the opening of a new tax collection office in peking.

Peng announced that a new draft of the marriage law presented to the congress would raise the legal age for marriage from 20 to 22 for men and from 18 to 20 women "to take account of actual conditions."

In practice, most local factory and commune officials, who issue marriage licenses, have been reluctant to do so unless the man was at least 25 and the woman 23. Peng today supported such restraint and said the new law did not mean "individuals were required to get married as soon as they reached those ages."

Late marriage has been encouraged as a way of cutting down the Chinese birth rate. Peng said the new law also included a clause saying a husband and wife "are duty bound to practice family planning." On the question of divorce, Peng appeared to take note of a number of recently publicized cases in which a spouse, usually the wife, has had trouble divorcing an abusive mate because of pressure from neighbors who are invited to comment in Chinese divorce courts.

"Since it is still a very long period since the feudal marriage system was abolished, Peng said, "some people are not sympathetic to the party who applies for the divorce and the courts have been strict."

Peng said the new law would say that divorce must be granted "in cases of complete alienation of mutual affection," which he said would help avoid forcing "people to maintain broken family relations and live in torment the rest of their lives."

The tax on joint ventures, which may be excused for some projects in their first three years, will probably be welcomed by American businessmen, since they may use it as a cedit against their U.S. taxes.

The new taxes reveal China generaly making its way into the world of modern finance, trying to earn more foreign exchange income while not really involving its own population.

In other developments, the Chinese press quoted leading generals as supporting an announced move to cut the defense budget by about $2 billion, although analysts say it is unclear just how much, if at all, total defense always will actually be reduced. One Army leader quoted defending the cuts was Xu Shiyou, who has been mysteriously absent from Peking for several months but has appeared at this congress.

In another speech at the congress session, Supreme People's Court President Jiang disclosed the results of a colossal review of more than a million criminal cases arising during the tumultuous Cultural Revolution from 1966 to 1976.

Jiang said many of the "appalling" cases were reversed, including 175,000 of 270,000 cases in which people had been charged with being counterrevolutionaries.

He said 76,000 of 860,000 ordianry criminal cases also were reversed.

Among those now judged wrongly accused during the late Mao Tsetung's attack on party bureaucrats were Jiang himself and Peng, who has assumed major responsibility for reforming China's legal code after being labeled one of the great villians of the Cultural Revolution.

Meanwhile, China's chief prosecutor, Huang Huoqing, decried today the effect the Cultural Revolution had had on youths, who emulated the lawless was of Mao's Red Guards. He said in the first half of this year "more than 84,000 criminals," had been arrested, better than 50 percent for "grave crimes." This had "deflated the arrogance of the criminal elements," he said, and "since March of this year, the number of criminal cases had dropped month by month."

Although his report did not appear to mention it, the drop in crime occured as Chinese authorites dispatched thousands of idle and trouble-making youths to labor camps, using a legal loophole that requires no court hearings in such cases. Huang said there continued to be resistance, however, to investigation of cases involving relatives of government officials.