Despite the assurances of Secretary of State Alexander M. Haig Jr. on U.S. policy toward Taiwan, China warned the United States again today that future arms sales to the Chinese Nationalists there would seriously jeopardize U.S. relations with Peking.

The official New China News Agency said in an authoritative commentary that the Reagan administration's declarations that it would sell Taiwan defensive weapons "cannot but cast a shadow over the development of Sino-American relations."

If the United States yields to the Taiwan lobby and goes through with the sales over Peking's protests, the commentary continued, then "Sino-American relations cannot even be preserved as they are today, to say nothing of development."

Peking's reiteration of its opposition to arms sales to Taiwan put the Reagan administration on notice that as far as China is concerned, it is still on probation on this issue despite the assurances Haig gave the Chinese during his four-day visit this week.

Haig had sought after the talks to portray American and Chinese differences on the issue as an agreement to disagree, telling a Peking news conference Tuesday that each side and explained its position and had understood that of the other.