President Reagan has decided to make an unusual appearance before a joint session of Congress next Wednesday night to seek support for his sagging Central American policy and attempt the rescue of his endangered military aid package for El Salvador.

Reagan was described by aides yesterday as believing that only a nationally televised speech, in which he will be appealing to the country as well as Congress, can rescue his $110 million aid request for the Salvadoran military after recent House committee votes to cut at least $50 million from it.

The aides said Reagan wanted to address a joint session because his past speeches on Central America did not receive the television coverage he sought.

"If history is written, God forbid, that Central America has gone communist and Mexico is flooded with refugees, they're not going to say that Ronald Reagan didn't do his dead level best to prevent it," said one administration official yesterday.

White House national security affairs adviser William P. Clark and chief of staff James A. Baker III, who have been feuding over defense spending, agreed yesterday that it is "critical" for the president to speak out to win congressional approval of the military aid for El Salvador, officials said.

One official said it was "absurd" for Congress to "blithely approve $450 million extra for Israeli aid and not give $50 million needed in our own back yard."

The House also has delayed for more than a month Reagan's request to transfer the remaining $60 million in additional military aid for El Salvador from other foreign aid accounts.

In addition, Democratic members of the House Intelligence Committee are considering new restrictions on the administration's financing of covert operations against the leftist government of Nicaragua, which has been accused by Reagan of shipping arms to leftist guerrillas in El Salvador.

Some critics have said the administration is violating an existing congressional prohibition against CIA activities aimed at overthrowing the Nicaraguan government.

After questioning Secretary of State George P. Shultz for three hours in closed session last night, the House committee is expected to decide today whether to restrict CIA activities further.

Reagan's address to Congress is to focus on the $110 million aid package. Senior administration officials are known to be frustrated with the congressional debate on Central America, which has instead focused on the legality of U.S. covert operations.

The president and his top foreign policy advisers are concerned about increased Cuban and Soviet influence in the region, which they see as a direct threat to U.S. interests.

"We will continue to push for the El Salvador aid package," White House spokesman Larry Speakes said yesterday. "We think it's essential this country, which is pledged to democratic principles and reforms, be provided with this . . . aid to provide a shield so they can protect themselves from people who obviously want to destroy their progress."

Reagan acknowledged for the first time last week that the United States was involved in covert operations against Nicaragua, but he insisted that their sole purpose was to stop the flow of arms from Nicaragua to El Salvador.

Several sources said this week that Democrats on the House Intelligence Committee may want to stop funding the covert operations because of evidence that the CIA is stretching its definition of "arms interdiction" to include the support of anti-Sandinista groups trying to overthrow the current government and whose aggressive military activities in Nicaragua cannot be defined as arms interdiction.

In announcing the speech plans yesterday, Speakes cited what he said was "foolproof evidence" that the Nicaragua regime was involved in gun running to Central American guerrillas. He referred to Brazil's grounding of four Libyan cargo planes loaded with a cache of arms and military equipment parts disguised as medical supplies.

The planes were bound for Nicaragua. Speakes did not say the arms shipment was bound for El Salvador, but stated it "very well could have been designed to aid those who would attempt to overthrow . . . democratic governments" in the region.

Speakes added, "This is foolproof evidence that outside forces are continuing to supply arms to the region in an attempt to upset the reforms that have been so eagerly sought by the governments there."