A House subcommittee has unanimously urged President Reagan not to sell Taiwan the FX or any other advanced combat aircraft now and to approach arm sales to China "with the greatest caution."

The letter, signed by all eight subcommittee members and delivered to the White House Friday, was written after a series of subcommittee hearings that explored U.S. policy toward China and Taiwan.

The Reagan administration is reviewing the U.S.-Taiwan strategic relationship. Reagan has indicated in the past that he might look favorably on upgrading Taiwan's air force with a new generation of aircraft to supplement the F5E, currently Taiwan's main combat plane.

"After the most careful consideration of the military threat to Taiwan, and the relative balance of power between [China] and Taiwan, during which we had a particularly useful hearing in executive session by the leading analysts of these precise questions from the intelligence community, we have come to the conclusion that it would be a mistake to sell the FX, or any other advanced combat aircraft to Taiwan, at this time," the letter said.

It added that every witness who testified "agreed that military tensions in the Taiwan straits were at an all-time low."

After concluding that the FX is not needed to meet any existing threat to Taiwan, the letter added the sale "would seem to make even less sense policically than it does militarily."

It might jeopardize U.S.-China relations which, in turn, would hamper the United States' ability to counter the Soviet Union, the letter said.

Chairman Stephen J. Solarz (D-N.Y.) and the other members of the Asian and Pacific affairs subcommittee of the House Foreign Affairs Committee cited the possible reactions of the Soviet Union and of U.S. friends in Asia as the reasons to be cautious about selling arms to Peking.

The letter was written just days after Secretary of State Alexander M. Haig Jr. announced a shift in U.S. policy that dropped the barriers to selling arms to China.

China lacks the foreign exchange to purchase much weaponry, but a sale that did little to enhance China's military capability still could be particularly provocative to Moscow and "would also be a source or real concern to our friends in the region," the letter said.

Any weapons systems approved for sale to China should be defensive so as to pose a threat to U.S. allies in Asia, the letter added.

The president, who is at Camp David, had not yet seen the letter, a White House official said yesterday. The official added, "We welcome receiving these congressmen's views and will study them as part of our policy process."