Despite official statements strongly opposing U.S. arms sales to Taiwan, China's leaders are expected to take a more flexible stance during next week's visit here by Secretary of State Alexander M. Haig Jr., influential Chinese sources said today.

Sources close to China's foreign policymakers said Peking is ready to tolerate continued U.S. weapons sales to Taipei so long as they do not exceed the sophistication or volume of arms currently sold by Washington to Taiwan, valued at between $700 million and $800 million per year.

China would respond adversely, however, if Washington decided to sell new offensive weapons, such as fighter planes, which could strengthen the resolve of Taiwan officials to resist Peking's recent diplomatic efforts to bring about a peaceful reunification of the island and the mainland, the sources said.

Since the Reagan administration took office, China has consistently criticized U.S. arms sales to Taipei as interference in Chinese domestic affairs and a violation of the 1979 Sino-American normalization treaty recognizing Peking as the sole, legal government of China.

Without ever publicly defining what it considers to be acceptable levels or kinds of arms sales, Peking repeatedly has warned it would make a "strong response" if the United States continued to sell weapons to Taiwan, especially the improved figher jet Taipei is now seeking.

Diplomats here said the more tolerant views privately expressed today by Chinese sources suggest that the nation's top policymakers may still be debating how best to resolve the sticky arms sales issue that is certain to be a centerpiece of the Haig visit, which begins Sunday.

The diplomats stressed, however, that this view seems to be less dominant than the hard-line positions stamped out in official statements, including a Foreign Ministry comment issued today that again threatened unspecified reprisals if Washington continues its arms sales "in defiance of our repeated, vigorous opposition."

A Chinese source expressing the more accommodating line explained it as a "realistic approach" to the arms sale question considerig the pressures on Reagan from the pro-Taiwan wing of the Republican Party and the president's campaign commitment to up-grade relations with the island government.

Last week, in a move intended to demonstrate accommodation on the U.S. part, the administration decided to allow China to buy defense-related technology from the United States. The Chinese source said that was a positive development for bilateral relations, but should not be seen as a way to appease China so that Washington can sell new weapons to Taiwan, he added.

In its statement today, the Foreign Ministry said that China "would rather not buy any U.S. weapons than agree to the continuous U.S. interference in China's internal affairs by selling weapons to Taiwan."

The source describing the more flexible posture noted, however, that China has tolerated U.S. arms sales to Taipei since Peking and Washington normalized relations in 1979. If sales remain similar to those since normalization, he said, he believes Chinese leaders would be willing to accept that status.

"If we wouldn't accept the sales," he asked, "how could there have been normalization between China and the United States?"

In the normalization agreement, the United States recognized Taiwan as a part of China but insisted on the right to maintain unofficial relations with the island, which the United States had for 30 years recognized as the true government of China.

Congress subsequently passed a law called the Taiwan Relations Act authorizing Washington to continue weapon sales to Taiwan. Not only did Peking not concur, it forcefully opposed the act, claiming it violated the spirit of the normalization treaty.

Early this year China downgraded its diplomatic relations with the Netherlands in reprisal for a Dutch agreement to sell two submarines to Taipei. The source said Peking took that drastic step, however, because the weapons sold by the Dutch are offensive and could embolden the Nationalist Chinese leaders.

Like other well-placed Chinese sources, he counseled Chinese patience in dealing with the touchy issue of U.S.-Tainwan policy for fear of stirring up what many here fear could be a right-wing surge in U.S. politics that could result in a new wave of McCarthyism and anti-China sentiment.

For that reason, he predicted that Chinese leaders would not press Haig for a clarification of the new administration's Taiwan policy, which has been the source of great controversy since Reagan first raised the possibility of upgrading relations with Taiwan during his campaign.