A breakthrough on the knotty issue of American arms sales to Taiwan is possible but not probable during Vice-President George Bush's forthcoming visit to China.
This is the cautious view of Western diplomats here. China is highly unlikely , these diplomats believe, to budge from its insistence that the United States agree to a definite time limit on further arms sales to Taiwan.
Mr. Bush knows the Chinese position, but is coming to Peking anyway, first because President Reagan asked him to, and second because Washington is said to feel that after long months of difficult talks between professional diplomats, it might be useful to have the American case presented to the Chinese leadership by the administration's most senior political leader next to the president himself.
Mr. Bush, who arrives in Hangzhou, China, May 5 and in Peking two days later, knows China's paramount leader Deng Xiaoping from the early l970s when he served here as chief of what was then the United States Liaison Mission.
Mr. Deng refers to Mr. Bush as an ''old friend of China,'' but this will not deter him from some pretty blunt speaking once he gets down to business.
The toughest Chinese suspicion Mr. Bush will have to try to dispel is that, despite lip service to the concept of ''one China,'' Washington wants to keep Taiwan separate from the mainland as long as it possibly can, and that arms sales to Taiwan are the principal means of ensuring this objective.
The official American position is that Washington has no objection to the reunification of Taiwan with the mainland so longas this goal is achieved peacefully, by negotiation, not by war. Many Chinese simply do not believe this contention.
Officially, Peking answers that, by sovereign right, China alone determines how it liberates Taiwan. It says that, since the United States has already acknowledged that China is one and that the People's Republic is the only legitimate government of all China, to continue supplying arms to Taiwan is a violation of China's sovereignty.
Informally, the Chinese say that they have in fact offered to negotiate with Taiwan many times--that a generous nine point offer allowing Taiwan de facto autonomy and even its own armed forces still stands.
But Taiwan has rejected every gesture, and Peking feels Taiwan will never have any incentive to start talking as long as it has an open ended commitment of arms supplies from Washington.
A commentary in the People's Daily newspaper May 2, obviously prepared with Mr. Bush's visit in mind, says ''the problem of US arms sales to Taiwan has reached a stage where it must be solved. The time bomb of the arms sale issue which consti-tutes a threat to Sino-US relations is created by the United States and can only be defused by Washington.''
Earlier, Vice Premier Bo Yibo told a visiting American business leader that although China had not downgraded relations with Washington over a $60 million sale of military spare parts to Taiwan announced by the Reagan administration last month, it would countenance no further sales of that sort.
''There is no question--and he was absolutely unanmbiguous about it--there would be a reaction'' if any further military sales occur, said the American, Christopher H. Phillips, president of the National Council for US-China Trade.
The Reagan administration is eager to maintain the strategic partnership with China forged by the previous Carter administration against what both sides saw as Soviet global expansionism.
Although in rhetoric Mr. Reagan has been more supportive of Taiwan than was his predecessor, in actual provision of arms he has been far more circumspect. The $60 million spare parts deal is the only major new arms sale to Taiwan authorized by the Reagan White House, compared to over a billion dollars worth supplied by Mr. Carter.
Mr. Bush will probably put this point to the Chinese with all the persuasive powers he can muster, as well as the argument that to pledge an explicit cutoff date for arms sales to Taiwan would not only destabilize Taiwan but also contravene the Taiwan Relations Act passed by Congress in l979.
But Mr. Deng has made the arms sales question a matter of principle involving China's sovereignty over Taiwan. The deadlock cannot easily be loosened, but Mr. Bush means to try.