Key figures in the formulation of a new national immigration policy disagreed today on the worth of a "guest worker" program being drafted by the Reagan administration.

Sen. Alan K. Simpson (R-Wyo.), Senate immigration subcommittee chairman, said he had "serious concerns" that without better controls on all illegal immigration, Mexicans wanting to come here would not bother to enroll as "guest workers."

But Simpson's opposite number in the House, Rep. Romano L. Mazzoli (D-Ky.), was more optimistic, saying he thought "the guest worker program may have a chance. . . with proper [legislative] language and proper size."

The "guest worker" proposal is a key element in an emerging package of immigration law changes designed to address what speakers at a San Diego City Club conference here called one of the most crucial social problems of the 1980s.

More than a million immigrants a year, both legal and illegal, are entering the country, and some critics like former labor secretary Ray Marshall blame them for as much as half of the nation's unemployment problem.

Many politicians and businessmen, however, say Mexican workers provide the only ready supply of labor for heavy physical work, such as harvesting crops, that American workers no longer are willing to do. U.S. Ambassador to Mexico John Gavin told the conference the Reagan administration was close to presenting a pilot guest worker program that would allow temporary workers in excess of Mexico's immigration quota to enter the country.

Some administration officials have suggested as many as 500,000 a year might participate, but Simpson said he expected the initial proposal to Congress would allow for only 50,000.

This would expand a program that allows 20,000 to 30,000 Mexicans to enter the United States for temporary jobs, mostly in border American towns.

The new guest-worker program would require strict monitoring of the workers to make sure they were given adequate wages and working conditions, several conference participants said.

Gavin said some Mexican newspapers and academic figures have charged the United States with luring illegal aliens across the border and then paying substandard wages, with the threat of deportation if they complain.

The program, as described here, would also restrict the guest workers to parts of the country that had labor shortages, particularly fruit and vegetable growing areas. Simpson said he feared many Mexicans would decide "they just don't want to put up with that" and continue to cross the border illegally.

Michael Teitelbaum, a demographer for the Ford Foundation and frequent adviser to congressional committees, also criticized the guest worker concept. He said that the bracero program from 1942 to 1964, an earlier version of the guest-worker scheme, may have increased rather than reduced the number of permanent immigrants from Mexico.

"Where such programs have been tried, they have tended to yield real short-term benefits to a few and real long-term problems for many."

Several speakers here, which also included Colorado Gov. Richard Lamm, agreed, however, that a package of changes was necessary, including better patrolling of the border, penalties for employers who knowingly hired illegal aliens and an amnesty permitting illegal aliens already here to remain inside the country.