The Reagan administration appears to be walking a fine line between China and Taiwan by only slightly reducing the quantity of arms sold to Taipei. The administration told Congress in private last week that it estimates about amount is slightly less than the $780 million the administration expects will have been sold by the end of this fiscal year.
The difference of $20 million apparently satisfies the administration's 1982 pledge to reduce the quantity of arms sold to Taiwan gradually over an unspecified period of time. Some pro-Taiwan members of Congress are considering how to restore higher levels of arms sales.
During his January visit to the US, China's Premier Zhao Ziyang singled out the Taiwan question as the main obstacle to improved US-Chinese relations. The reduced arms sales thus could help remove one impediment to those relations.
Still to be announced, however, is the quality of this year's arms package for Taiwan, which the administration also pledged in 1982 would not exceed the level of previous years.
In the past, the Taiwanese have requested aircraft as advanced as the F-16A, as well as the less capable F-16/J79 and F-20A (formerly F-5G), according to a recent Heritage Foundation report. The Reagan administration concluded in February 1982 that no sales of advanced fighter aircraft to Taiwan were required because no military need then existed. Left open was the question of what would happen if China upgraded its military capabilities.
So far, defense has been at the bottom of Peking's list of modernization priorities. Sen. Frank Murkowski (R), who chairs the Senate's Subcommittee on East Asian and Pacific Affairs, was reported to say during his visit to Taiwan last month that if China develops all-weather fighters that could threaten Taiwan, then Taiwan should be able to buy similar fighters from the US.
Later this year, the US Defense Department will announce exactly what will be approved for arms sales to Taiwan. A document presented to Congress by the Defense Security Assistance Agency projects that Taiwan will have access on a cash basis to ''selected'' US defensive weapons. It does not specify what kind.