The Immigration and Naturalization Service has liberalized its interpretation of who is eligible for amnesty and, as a result, thousands of previously ineligible illegal immigrants will qualify for the unprecedented, one-year program, INS officials and lawyers said.

INS Commissioner Alan Nelson is expected to announce these policy changes at a news conference today. They have been implemented during the last two months in INS legalization offices nationwide.

Nelson also is expected to reveal a new policy involving illegal immigrants who at some point left the United States but returned to this country using a valid visa. The INS position on these illegal immigrants has been that they are not eligible for amnesty.

Nelson is not likely, however, to address the emotional issue of what the INS will do with family members of illegal immigrants who receive amnesty but are not eligible themselves because they entered this country after Jan. 1, 1982, sources said.

While immigration lawyers and immigration advocacy groups welcome the INS decision to relax some previously stringent guidelines for the amnesty program, they view the changes skeptically and question the agency's motives.

They said that more than a dozen lawsuits filed on behalf of illegal aliens challenging the INS administration of the amnesty program and the agency's need for more money to run it are the primary reasons for the INS changes.

INS officials strongly deny that the fear of litigation or a desire for more revenue is behind the changes, which they call "clarifications."

"We are not doing it just to get numbers," INS spokesman Duke Austin said. "If we didn't {relax the regulations} they'd say we are terrible, and if we do something good and that makes common sense, then we have to have an ulterior motive. No matter what we do, those guys will come up with a hidden agenda for the INS . . . . "

The immigration law signed by President Reagan last November includes the amnesty program for undocumented people who have lived here since Jan. 1, 1982, and also for the first time imposes sanctions on employers of illegal aliens as a way of curtailing illegal immigration. Amnesty is a four-step legal process that leads to permanent residency.

In the last two months, INS officials have announced the following changes in their interpretation of the new law:Diplomats and international employes who left their jobs before Jan. 1, 1982, and remained here illegally are now eligible for amnesty.

Foreign students who remained in this country illegally 30 days after completing their studies before Jan. 1, 1982, are eligible for amnesty.

Before the INS modified its position on student, diplomatic and international work visas, many undocumented immigrants had to prove that the INS was aware they were here illegally, a tough standard to meet unless the INS had made a notation in its alien file or had begun deportation proceedings against them. Undocumented immigrants who applied for political asylum before Jan. 1, 1982, will now be eligible for amnesty if their asylum petitions were denied or are still pending.

The changes, however, will not help the 50,000 undocumented aliens, most of them students, whose visas expired after Jan. 1, 1982, but who had been working illegally before that date. They still must prove that the INS knew they were here illegally to qualify for amnesty, and lawyers expect most of them will be denied amnesty.

"I think that any sense that they {INS officials} are becoming more liberal is misplaced," said Carolyn Waller, a lawyer with the aliens' rights law project of the Washington Lawyers' Committee for Civil Rights. "They are facing specific issues and in some cases they are taking the liberal option."

Michael Maggio, a well-known District immigration lawyer who is vice president of the American Immigration Lawyers Association, said he believes the INS has relaxed the guidelines for some amnesty applicants because the agency needs money for the financially ailing program, which is supposed to be funded solely by fees from applicants.

"They are becoming more generous so the program will pay for itself and they don't have to reach into their own pocket," said Maggio.

Initially, INS officials had said that 3 million to 4 million illegal immigrants would apply for amnesty, but now they expect the program to attract about 2 million people. Since May, when the program began, more than 600,000 immigrants had applied for amnesty.

Austin said he expects proceeds from the amnesty program to "be awfully close" to the $100 million to $105 million it will cost the government to operate it this fiscal year.

But some attorneys said they intend to play David to the INS' Goliath by encouraging some clients to apply for amnesty even though they know the INS probably will turn them down. Then, the lawyers said, they will appeal the INS decision and the case could ultimately be decided in court.