As U.S. congressional leaders wound up their talks here today, China launched its harshest criticism against the United States in months, charging it with "double-dealing" in supplying military equipment to Taiwan.

The official China Daily accused Washington of trying to "dictate the terms" of Taiwan's reunification with the Communist-ruled mainland despite earlier U.S. acknowledgements that the issue should be left to the Chinese to resolve.

"It is sheer mockery for the United States to profess interest in peaceful reunification of Taiwan while militarily supporting the Taiwan authorities' intransigence in holding on to the status quo," said the English-language newspaper.

Western diplomats said the attack--the sharpest since Peking and Washington signed an agreement in August aimed at defusing the Taiwan issue--apparently was timed to impress the congressional delegation headed by House Speaker Thomas P. (Tip) O'Neill, Jr.

O'Neill, speaking at an afternoon news conference, said the visiting group that includes two House committee chairmen was surprised by "the strong position the Chinese government took with regard to the Taiwan question."

"What is number one in one country is not number one in another country," said O'Neill. "On a scale of 1 to 35 in America, where would I place the Taiwan-Chinese situation? It would be hard for me to place it anywhere along the line."

If there was any question where Peking places the issue, China Daily answered it point-blank in its commentary today.

Although Washington formally shifted its recognition from Taipei to Peking in 1979 and acknowledged that the island is part of China, said the newspaper, U.S. officials still refer to Taiwan as an ally.

"What is this if not duplicity?" asked the commentary.

China Daily said "continued double-dealing" by Washington will further endanger Sino-American relations, which "are not what they should be for lack of fundamental trust."

The commentary recalled U.S. support for Nationalist Chinese forces before the Communist victory in 1949, blaming Washington for prolonging the civil war and causing more deaths and destruction than necessary.

Continued arms transfers to the Nationalists now ruling Taiwan have "delayed unification of the province with China against the will of the people on both sides of the Taiwan Strait," according to the newspaper.

Peking's criticisms have grown increasingly strident since the August communique in which Washington pledged to phase out arms sales to Taiwan and Peking promised to use peaceful methods of reincorporating the breakaway island.

Although the document was vaguely worded and open to varying interpretations, the Chinese leadership has been hostile to the Reagan administration's views of its obligations in the agreement.

President Reagan has assured Taiwan supporters by explicitly linking any cutoff in arms sales to peaceful reunification of the Nationalist stronghold with the mainland.

In meetings with congressmen this week, Chinese officials reportedly emphasized their position that U.S. military supplies make Taiwan's leaders more resistent to peaceful overtures.

Some of the sessions are said to have become heated when U.S. officials challenged the Chinese claim. Rep. Clement J. Zablocki (D-Wis.), chairman of the House Foreign Affairs Committee, said he told his hosts that U.S. arms sales help ensure peaceful reunification by deterring Peking's use of force.

The congressional delegation left today for sightseeing in other parts of China.