President Reagan abruptly canceled a White House signing ceremony Wednesday for an export-promotion bill -- too late, it turned out, to notify some of the invited guests.

The ceremony is now set for today, as the president tours Los Angeles harbor in full view of television cameras. The apparent intent is to deflect the impact of the high unemployment figures that are to be released today in Washington. The new law will mean more American exports and thus more jobs, the administration promises.

The sudden cancellation of Wednesday's White House ceremony surprised some of the bill's supporters in the export field who had been invited here. Speculation that today's signing is intended to blunt the unemployment figures spread beyond partisan Democrats.

"One would have to think that it has something to do with this being a jobs bill that could be a powerful counterpoint to what's coming out Friday," said an official of the U.S. Chamber of Commerce.

The bill, in fact, had bipartisan support and was strongly favored by the Carter administration, which was unable to spring it from Congress. Although Commerce Secretary Malcolm Baldrige testified for it, the bill had received scant attention from the White House until the president's Sept. 28 press conference, when Reagan listed it in a prepared statement as a job-creating measure that Congress was holding up.

Until then, the bill -- which passed both houses of Congress in the waning hours of the last session -- was little known outside the small coterie of specialists who follow foreign trade.

It is designed to make it easier for small and medium-size businesses to sell overseas by allowing export-trading corporations special exemptions from antitrust laws and letting banks join those corporations, which would act as middlemen in dealing with foreign buyers.

"By tapping the export potential of many thousands of small and medium-size businesses, we can help the United States regain the initiative in world trade," said Sen. John C. Danforth (R-Mo.), one of the bill's major sponsors. He and former senator Adlai Stevenson (D-Ill.) began pushing the legislation in 1979.

The bill's other major sponsor, Sen. John Heinz (R-Pa.), cited estimates that it could create 640,000 jobs within three years. This estimate is more optimistic than one offered by Baldrige, who predicted that exports stimulated by newly formed trading corporations would produce 320,000 jobs by 1985. He expects U.S. exports to grow by $27 billion because of the bill.

The Commerce Department says that less than 10 percent of the nation's 300,000 manufacturing firms now sell overseas and that 20,000 American companies now producing only for the domestic market could begin tapping the export market if they had help from professional trading companies.

Joel D. Honigberg, a long-time overseas trader who has his own Chicago-based export corporation, was one of the bill's supporters who came to Washington after receiving an invitation for Wednesday's signing ceremony.