VENICE, JUNE 8 -- President Reagan, praising Japan's efforts to stop the cut-rate dumping of semiconductors in foreign markets, today ordered a partial reduction of $51 million in sanctions against Japanese electronics exports in the United States.

The president said the 17 percent cut in the $300 million in sanctions imposed six weeks ago "is strictly proportional" to the partial withdrawal by Japan of trade practices that the United States found objectionable and that triggered the punitive measures.

Koji Watanabe, a high-ranking Japanese Foreign Ministry official present at a meeting today between Reagan and Japanese Prime Minister Yasuhiro Nakasone in which the reduction of sanctions was discussed, said Nakasone expressed his "appreciation" for the partial step even though he pressed for the abolition of all sanctions.

"It's much better than nothing and augurs well for the full removal of sanctions," Watanabe said. He added that Reagan's order would provide a psychological lift to Japanese citizens who believed that the imposition of sanctions was unfair.

The Reagan-Nakasone meeting preceded the formal dinner of the seven leaders of industrialized democracies that opened the 13th annual economic summit. During the meeting, Nakasone reiterated a pledge to Reagan that he would make an effort to push Japanese interest rates lower in a bid to stimulate the economy, it was learned.

In another bilateral session with West German Chancellor Helmut Kohl, Reagan pressed for further West German efforts to help global expansion. Kohl listened to Reagan's appeal but made no pledges, according to White House chief of staff Howard H. Baker Jr.

American officials are hoping to get a summit declaration that will commit the seven powers to undertake expansionary steps to sustain economic growth for some years ahead. But there are signs that the Kohl government is strongly resisting the pressure.

Baker later told reporters, "I think there'll be important news coming out of the summit."

In their meeting, Nakasone assured Reagan that Japan would work strenuously to bring about conditions in the semiconductor trade that will permit total elimination of the sanctions "as soon as possible."

In his public announcement today, Reagan expressed regret that the Japanese had not yet taken sufficient action to enable him to scrap the sanctions.

He made clear that the partial lifting of sanctions was undertaken with full knowledge that the conciliatory move may not be well-received on Capitol Hill. Influential Republicans and Democrats have argued that the total amount of sanctions should be retained until Japan has abandoned all dumping.

{Republicans and Democrats on Capitol Hill attacked the president's decision, Washington Post staff writer Stuart Auerbach reported.

{Senate Majority Leader Robert C. Byrd (D-W.Va.) said he "regretted" the president's action.

{Sen. Pete Wilson (R-Calif.) called the partial lifting of sanctions "a serious mistake" that "risks encouraging the Japanese to think that they can again continue in their own way at no cost." Added another Republican, Sen. John C. Danforth of Missouri, "The Japanese will view this as a return to business as usual."

{But Senate Minority Leader Robert J. Dole (R-Kan.), who last week urged the president not to lift sanctions, supported Reagan's position, saying it "is very limited and makes sense."}

Reagan said that while the United States sought to use sanctions to send "a clear message" to Japan to cease unfair trade practices, he intended to lift the measures as soon as possible in order to encourage free-trade policies.

"Japan is a major economic partner as well as a staunch friend and ally," the president said. He also praised Nakasone's personal intervention in seeking to halt the dumping and said, "We think that they ought to be rewarded for their effort."