President Carter yesterday postponed until March 31 action on a Civil Aeronautics Board proposal to reduce some of Japan Air Lines' flights into the United States in retaliation for Japan's refusal to approve the planned flights of a U.S. carrier.

In a letter to CAB Chairman Alfred E. Kahn, the president said he agreed with the board that Japan violated the U.S.-Japan air agreement by refusing to approve three flights a week by Flying Tiger Lines, Inc., a U.S. all-cargo carrier, between the U.S. and Singapore, with stops on Japan and Hong Kong. " . . . responsive action may properly be taken by the U.S. Government as a result," he said.

However, since negotiations with the Japanese resume here today on a new air services agreement, Carter said he would stay the board's order. "These discussions should take place in a positive climate," he said. "I am confident that the Japanese authorities, upon reconsideration during the negotiations, will remove the basis for the board's order by approving U.S. airline schedule changes."

He warned that he would consider removing the stay if the negotiations don't resolve the matter by March 31.

Carter was faced with a similar decision a week ago when the CAB had recommended he take action against British Caledonian Airways in retaliation for British failure to approve low fares proposed by Braniff Airways for its new route between Dallas and London. In that case, he also took similar action - saying he would consider retaliatory steps if the issue wasn't resolved by March 17 during negotiations with the British.

That strategy appears to have worked with the British. Although no announcements have been made about the progress of the talks, it is believed the British have agreed to accept the U.S. interpretation of the existing bilaternal and to approve low-fare proposals from Braniff and other airlines flying between the two countries.

Negotiations with the Japanese, which began in Tokyo last fall, are aimed at revising the 25-year-old agreement between the two countries at Japan's instigation. Little progress has been made in the several earlier sessions, according to both U.S. and Japanese sources.

The Japanese maintain the existing agreement, signed in the closing days of the American occupation after World War II, is full in inequalities that favor U.S. companies. They especially want access to more U.S. cities for JAL, which is half owned by the government. For its part, the U.S. government wants to assure passengers of lower fares to Japan than have been allowed so far as well as access to Japanese cities by U.S. charter airlines.