U.S. District Court Judge Joyce Hens Green yesterday dismissed a lawsuit by 29 members of Congress who contended that the Reagan administration acted illegally in sending U.S. military advisers and other assistance to El Salvador early last year.

The House members had argued that the administration violated the War Powers Resolution, which requires the president to report immediately to Congress when U.S. troops are dispatched to areas of hostility without a declaration of war.

The lawsuit also contended that the administration had violated the Foreign Assistance Act of 1961, which prohibits security aid to countries with a consistent pattern of human rights violations.

In a 16-page decision, Green ruled that Congress, not the court, must resolve the complex question of whether U.S. forces in El Salvador are involved in a hostile or potentially hostile situation, which would trigger provisions of the War Power Resolution.

There could be circumstances, such as the huge loss of lives during the Vietnam war, under which a court could conclude that U.S. forces were clearly involved in hostilities, Green said in her opinion.

In the Salvadoran case, however, Green said, she is faced with "a dispute as to whether a small number of American military personnel who apparently have suffered no casualties has been introduced into hostilities or imminent hostilities."

"The subtleties of fact-finding in this situation should be left to the political branches," she wrote.

The War Powers Resolution, passed by Congress in 1973 near the end of the Vietnam war, requires that the president report to Congress within 48 hours of the introduction of any forces to a hostile area. If Congress takes no action within 60 days, the president is required to withdraw those forces.

Green noted that no such report had been filed by President Reagan and that Congress "has taken absolutely no action" to show that it believes that his decision is subject to the War Powers Resolution.

Even if the fact-finding problem could be resolved, Green said, she could at most order the president to file a report to Congress. The 29 House members had asked Green to order immediate withdrawal of U.S. forces from El Salvador.

On the issue of human-rights violations, Green noted that Congress specifically addressed that issue last December when it approved aid to El Salvador provided that the president certify every three months that the Salvadoran government is trying to end such violations.

Two such reports have been made and Congress has taken no action to stop aid to El Salvador, Green noted. If those who brought the lawsuit disagree with the reports, Green said, their dispute is "with their fellow legislators" who took no action to stop aid.