Secretary of Commerce Malcolm Baldrige yesterday warned the Japanese government that unless specific actions to open Japanese markets to American business are forthcoming in the next six months, the Reagan administration will have to rethink its opposition to legislation restricting Japanese imports.

Saying the Japanese are mistaken in "thinking the United States is a paper tiger" that will not retaliate against unfair trade practices, Baldrige said "the impetus is building inside the administration" for supporting such measures as a "reciprocity" bill that would condition U.S. trade practices on Japan's.

In an interview with a group of reporters, Baldrige also revealed that the Reagan administration is studying possible revisions of antitrust laws to make it easier for U.S. firms to cooperate on research and development projects to meet foreign competition.

His remarks, on the eve of the ministerial conference on trade practices beginning this week in Geneva, were clearly intended to signal a harder administration bargaining position, after President Reagan in a weekend speech stressed America's opposition to protectionist practices.

Baldrige condemned the most widely sponsored piece of legislation in this area, the "domestic content" bill, aimed chiefly at Japanese auto manufacturers. It would require companies that sell large volumes of products in this country to make progressively larger percentages of them in U.S. plants.

While criticizing this as a "second-rate approach," Baldrige said he and his Cabinet colleagues were "getting tired" of trying to persuade Congress not to retaliate against Japanese practices restricting American companies' access to Japanese markets.

He said the Japanese government has pledged repeatedly to change those practices but has yet to follow through on "about 15 major actions we've asked them to take -- concrete, specific steps that will show up in the balance of trade very quickly."

Unless such action is forthcoming in the next six months, Baldrige said, the administration will either recommend its own legislation or acquiesce in supporting one of the bills before Congress to restrict Japanese imports.

The Commerce secretary was not specific about the antitrust changes under consideration, but noted that some of the existing laws barring mergers or pooled research were drafted 70 years ago, when the main threat to U.S. businesses was monopoly of domestic markets. In terms of today's international competition, he said, such laws may be damaging American industry. The question is scheduled to come before the Cabinet committee on commerce and trade early next year, he said.