The Civil Aeronautics Board acceded yesterday to a State Department request to speed up processing of an application that would provide the first unrestricted nonstop passenger flights from the United States to China in 30 years.

At issue are three nonstop charter flights from San Francisco to Shanghai to be operated by Pan American World Airways on behalf of China and its airline, and for Pan Am to operate three more of its own. The first flight is scheduled to depart San Francisco on Dec. 17.

Yesterday's board action came in response to a petition from the State Department asking the board itself to review action taken by the staff last week denying Pan Am and China an expedited timetable for the normal board procedures. Their application had asked for a board decision by Sept. 15 because after that date either party could withdraw from their agreement setting up the flights.

The State Department, in a filing submitted by James R. Atwood, deputy legal adviser, said it was aware of no U.S. airline application for charter service to China that had been turned down since diplomatic relations were established at the beginning of the year.

Although Vice President Walter F. Mondale's current visit to China was not mentioned in any official documents, State Department officials were said to be upset that the CAB staff action of last week would be an embarrassment while Mondale is in China.

The vice president is expected to discuss, among other things, a U.S.-China bilateral air services agreement that would be needed before regularly scheduled air services could be resumed.

In another filing with the CAB yesterday, the General Administration of Civil Aviation of China brought up the board's staff concern that other U.S. carriers may not be permitted to perform charters to China and said, "There is absolutely no basis for this speculation."

In another development, the CAB disapproved a proposal by Air Canada to offer a discount "seat sale" this fall for U.S.-Canada routes of at least 1,000 miles in length. The board's decision is subject to President Carter's approval.

The fare, with various restrictions, would reduce travel to levels 46 to 66 percent below normal economy fares.

The CAB said it does not object to the fare proposal on its merits, but felt obliged to suspend it because the Canadian government has recently rejected a number of low-fare proposals from U.S. airlines.