From the sound and fury of China's propaganda mill since Alexander M. Haig Jr. left Wednesday, one might think the American secretary of state had eaten more crow than Peking duck during his three days of talks here with Chinese leaders.

On the day Haig left, Peking broadly hinted that it was rethinking its anti-Soviet policy that has pushed China closer to the United States since 1979. Then it warned again of the dire consequences that could arise from President Reagan's continued support of arms sales to Taiwan.

This hard-line talk, however, belies the more relaxed approach Peking apparently adopted in talks with U.S. officials. Despite threats before the visit, Peking did not insist on a cutoff of U.S. arms to Taiwan as a condition of further developing Sino-American relations.

While still grumbling about U.S. arms transfers to Taiwan when the talks ended, Peking nevertheless moved closer to a military relationship with the United States by agreeing to send a high-level delegation to Washington in July to discuss purchases of U.S. weapons.

"When dealing with the Chinese, you have to distinguish between their position and the implementation of their position," observed a diplomat who has negotiated with Peking for years.

The diplomat said Peking often stakes out extreme positions on touch issues, then softens its stand if compromise serves its interests best; nevertheless, it continues to state the origtinal position as official policy.

A Chinese official said privately just before Haig's visit, "There is a difference between theory and practice. We would like in theory to have no [U.S.] sales of weapons to Taiwan. But we live in a practical world."

Just what persuaded China to take a more flexible stance during the talks remains undisclosed. Chinese sources indicated that while Haig refused to rule out future arms sales to Taiwan, he stressed that Washington would do nothing to sacrifice the larger strategic interests it shares with Peking, especially containing the Soviet advance in Asia.

Haig also gave assurances, according to Chinese insiders, that despite sporadic pro-Taiwan statements by administration officials, Reagan's China policy is guided by the 1979 Sino-American normalization agreement that recognizes Peking as the sole legal government of China and Taiwan as part of the mainland.

"The Taiwan issue was handled in such a fashion as to enable the relationship to keep moving ahead without removing the central issue," according to a participant in the talks.

Despite the focus on the issue, the question of U.S. arms sales to Taiwan was not really negotiated in the talks, participants said. Instead each side merely presented its position, the Chinese reiterating their opposition to all arms transfers to Taiwan while the Americans tried to make a distinction between defensive and offensive arms.

China is said to hve stressed the sensitivity of its relations with Taiwan, arguing that the United States should do nothing to interfere with Peking's plans to reunify with the island peacefully.

Ironically, Chinese sensitivites were aggravated shortly after Haig announced in Peking what amounts to the biggest breakthrough in Chinese-U.S. relations since normalization two years ago: they U.S. decision to sell lethal arms to China on a case-by-case basis.

At a press conference in Washington, Reagan said four hours later that he still had good feelings for his old friends on Taiwan and plans to live up to the congressional act allowing U.S. sales of defensive arms to the nationalist Chinese.

Peking, which veered from its official line by consenting to the important bilateral development even though the Taiwan isue had not been settled to its liking, suddenly restated its hard-line position.

"Their position is entirely consistent," said a Western diplomat. "When they feel they can give up ground without losing vital interests, they act with flexibility. When they think their interests are being trampled upon, then they scream."

First Peking published an article in the Communist Party newspaper calling for peaceful settlement of the Sino-Oviet border dispute. Diplomatic specialists concluded that the proposal was aimed less at the Soviets than the Americans, reminding Washington that Peking has other diplomatic options.

Then the official New China News Agency published a commentary apparently in response to the Reagan press conference.

After warning that continued arms sales to Taipei "cast a shadow" on Chinese-U.S. relations, the commentator aimed his remarks at Americans who contend Washington has co-opted China by offering to sell it weapons.

Recalling how China broke ties with the Soviet Union in 1960 despite its heavy dependence on Soviet advisers and aid, the news agency said that break manifests the "daring spirit of the Chinese people" to defend their independence.