The Japanese government presented a new plan today for curbing auto exports to the United States and claimed that it had won the general approval of special trade representative William Brock.

Other sources said Brock did not express actual agreement with the new plan, but they acknowledged that a new Japanese package raised hopes that exports would cut back significantly.

Japan is trying to produce a formula for cutting exports that would satisfy the American Congress -- which is threatening to pass strict quota legislation to take the pressure of Japanese car sales off the American auto industry -- but cause the least possible harm to the domestic industry.

According to Japanese sources, the plan calls for reducing exports this year to a level of 1.68 million cars, compared with the 1.82 million exported last year to the American market.

Next year, the same level of 1.68 million would be set, but Japan would be permitted to exceed that level somewhat if the U.S. auto market improved substantially.

In the third year of the plan, the level of exports would be reconsidered.

That version of the new offer, however, does not appear to meet the limitations spelled out in legislation introduced in Congress by Sen. John Danforth (R-Mo.).

Danforth's bill would set a strict limit of 1.6 million exports a year for the entire three-year period, and the senator has said that anything less would not be acceptable to Congress.

Brock has said he did not come to Japan to negotiate an agreement. However, sources said he would advise the Japanese on what chances their plan might have of forestalling action by the U.S. Congress.

Brock met this morning with Rokusuke Tanaka, minister of international trade and industry, and government officials later claimed a broad acceptance of the plan had been attained. An American source described this morning's session as "upbeat" and added, "What came up this morning was significantly different than what we got yesterday."

That was in sharp contrast to the reaction last night of U.S. trade officials, who gave a gloomy assessment of the first round of meetings between Brock and Tanaka.

U.S. officials also said last night that the Japanese, led by Tanaka, had attempted to lay down one condition that was not acceptable: that acceptance of their plan should bind the Reagan administration to block, by veto if necessary, any further congressional attempt to impose quotas.

But another major roadblock may have been removed by a U.S. Justice Department comment that seemed to have eliminated the danger of antitrust action against Japanese auto firms.

Meanwhile, Japan's auto makers clung to their public insistence that they would not voluntarily reduce exports. Takashi Ishihara, president of Nissan Motors of the auto makers' trade association, told Tanaka earlier yesterday that he could not agree to cut below the level of 1.82 million cars exported to the United States last year.

A major concern here has been that any voluntary agreement to limit exports would draw antitrust suits either from the U.S. Justice Department or from the Japanese auto dealers in America if some degree of government coercion forces the restraint.

A Japanese representative this week informed the Justice Department what degree of coercion is being planned. According to the U.S. trade official, the Japanese plan was considered "compliant" with the antitrust law.

The government has not revealed what that restraint will be. It is known to be considering a strict monitoring plan in which each auto maker would be told to reduce exports by a fixed amount.

Government officials continued to talk of a quick solution to the impasse, which has dragged on for weeks. Prime Minister Zenko ysuzuki said this week the problem will be solved before he meets with President Reagan in Washington next Thursday. Brock will meet Suzuki briefly before leaving.

President Reagan has told Japanese officials it would be difficult politically to veto the congressional quota bill, but a U.S. trade official said here yesterday that no decision has been made on a potential veto.

The official said the president's decision might depend on what actions the Japanese goverment takes to attain export restraints. If Japan offers to do very little, he said, the congressional reponse would be anger and it would make a veto even more difficult.

He quoted congressional leaders as saying the bill introduced by Danforth would be enacted promptly if Japan does not act first to restrain exports.