What the US ought to tell Mr. Deng

After months of debate and indecision inside the government, the Reagan administration has finally decided against the sale of advanced aircraft like the FX to Taiwan, but would allow Taiwan to continue to buy more F-5E fighters. The decision was conveyed to the Chinese leaders recently by Mr. John Holdridge, US assistant secretary of state for East Asian and Pacific affairs, who went to Peking on a secret mission. The compromise decision, reached under intense Chinese pressure, has failed to mollify the Chinese leadership, however. During and after Mr. Holdridge's visit, communist officials have reiterated Peking's strong objection to the sale of any American weapons to Taiwan and warned the US of a diplomatic breakdown if the issue is not resolved.

By rejecting sale of the FX to Taiwan, President Reagan has shown considerable sensitivity to the feelings of Peking leaders, even if the move inevitably incurs criticism from some members of Congress who are his key supporters. Yet Peking's continuous intransigence could provoke a US-Sino diplomatic crisis that neither side desires. It is high time that we point out to the Chinese that:

(1) President Reagan has already met China's demand more than half way. As the US President, he has the constitutional obligation to carry out provisions of the Taiwan Relations Act. No president, not even Jimmy Carter, could agree to Peking's demand that the US end all arms sales to Taiwan. Don't push Mr. Reagan too hard, he is not someone who easily knuckles under.

(2) The Reagan administration wants to stand firm against Moscow and project strength and decisiveness in American foreign policy globally. An active, strong and reliable America is in China's best interests. Hence, Peking should not expect the US to bow to China's pressure and still be able to stand up to the Soviets.

(3) For Peking to place the issue of arms sales to Taiwan ahead of the global strategic cooperation against Moscow, especially during the Polish crisis, would make a mockery of all the talk of US-China parallel interests and strategic partnership. This is an opportune moment for Chinese leaders to show their statesmanship and prove the ''strategic imperative'' of a close US-China relationship, lest China's credibility as a valuable partner against Soviet hegemonism be destroyed.

(4) The US does not seek Taiwan's permanent separation from China, nor are the arms sales to Taiwan the cause of such division. The truth is that Taiwan does not wish to be reunified with the mainland, so it has tried to obtain arms to defend itself against a possible use of force by the communists. Hence pressuring the US to end arms sales to Taiwan is not really helpful to the cause of reunification.

(5) China's reunification proposal is a positive step - indeed a highly skillful diplomatic move, as it makes Peking look flexible, reasonable, and magnanimous and puts the nationalists in Taipei on the defensive. The offer to let Taiwan keep its socio-economic system and armed forces is even more reassuring if Peking can convince people on Taiwan of its sincerity. In this connection, Peking's objection to Taiwan's purchase of arms from the US is an enormous political mistake, for it makes the reunification offer an empty gesture. Unless people on Taiwan can feel secure about their life, freedoms, and high living standards, the nationalist government will never respond to Peking's peace gesture.

(6) Besides, if the US ceases to provide selective defensive weapons and Taiwan turns elsewhere for its arms, or if in desperation for its survival Taiwan seeks to develop nuclear weapons, the consequences could be a lot worse.

Parris Chang, professor of political science and chairman of East Asian studies at Pennsylvania State University, is author of ''Power and Policy in China'' and numerous other writings on Asian affairs.