The Reagan administration appears to be boycotting an event this July commemorating the 10th anniversary of the first and only linkup in space of American astronauts and Soviet cosmonauts.

The ceremony is to be part of a conference on Mars, sponsored by two private U.S. organizations, the Planetary Society and the American Institute of Aeronautics and Astronautics. Participants at the conference, to be held July 16 at the National Academy of Sciences, will discuss how the two nations plan to explore Mars and how they might explore it together.

Although the White House, State Department and National Security Council have been invited to the event, at this time none plans to send representatives, according to the conference organizers.

The National Aeronautics and Space Administration will send representatives to the conference, but Peter Smith, a spokesman for its International Affairs Division, said the space agency has no plans to mark the 10th anniversary of the Apollo-Soyuz flight. "That could change next week," Smith said, "but NASA has no plans to celebrate the anniversary."

One source with the organizing groups said the White House had considered giving its stamp of approval to the 10th anniversary celebration until the slaying of Army Maj. Arthur D. Nicholson Jr. by a Soviet soldier in East Germany on March 24. "Nicholson's murder changed everything," said the source. "The White House lost all interest in the anniversary."

A White House spokesman declined to comment.

Soviet cosmonauts Alexei Leonov and Valeriy Kubasov and American astronauts Thomas P. Stafford, Donald K. (Deke) Slayton and Vance D. Brand have been invited to attend, but the Soviets have not accepted yet. The five men linked their spacecraft for two days in 1975, a milestone then hailed by President Gerald R. Ford as the start of a "new era of international cooperation."

The U.S.-Soviet Space Cooperation Agreement signed during the Nixon administration was allowed to lapse in 1982, however, when martial law was declared in Poland. A resolution to renew the agreement was passed by the Senate and signed by President Reagan in October, but no action has been taken since then.

Stafford, now a retired Air Force lieutenant general, visited Soviet Ambassador Anatoliy Dobrynin this week, ostensibly to extend him an invitation to attend the conference. Stafford's visit was described also as an attempt to placate the Soviets, who reportedly are upset by the administration's apparent lack of interest in marking the historic space flight.

Sen. Spark M. Matsunaga (D-Hawaii), who sponsored the resolution to renew the space cooperation agreement, said he thinks the Reagan administration is making a serious mistake by ignoring the event.

"I don't see how failing to commemorate one of the rare instances of decency in Soviet behavior can be viewed as a meaningful response to the brutal killing of Maj. Nicholson," Matsunaga said. "Besides, the Apollo-Soyuz linkup in space was a heroic feat whose heroes risked their lives and deserve to be honored."

Matsunaga, who is to speak at the event, said he plans to call for a joint manned mission to Mars by the United States and the Soviet Union. Also participating in the conference will be NASA Administrator James M. Beggs, astronaut Sally K. Ride, Cornell University's Carl Sagan, former astronaut and former U.S. senator Harrison H. Schmitt (R-N.M.) and Bruce Murray, former director of the Jet Propulsion Laboratory.