The Immigration and Naturalization Service, in an effort to avert agricultural labor shortages this summer, announced yesterday that it will allow alien farm workers to enter the United States from Mexico based on their word that they meet requirements of the new immigration law.

The INS has been using a cumbersome process in which Mexican farm workers were required to provide documentation to the U.S. Embassy in Mexico City, a procedure expected to take as long as two to three months and stretch through most of the U.S. harvest season.

To be eligible for admission and eventual legal status, the workers must prove that they worked for at least 90 days harvesting perishable U.S. crops in the year ending May 1, 1986.

Under heavy pressure from western growers and members of Congress, INS Commissioner Alan C. Nelson announced yesterday that, starting Wednesday, the workers can also be processed at the border-crossing at Calexico, Calif., about a two-hour bus ride from Tijuana, Mexico.

If necessary, the INS may also set up a processing station in Texas for farm workers, he said.

Nelson said the workers would be admitted without documentation, but they would have to be "relatively specific as to where they worked and how long." He said the workers would be required to pay the $185 fee charged to all aliens seeking legal status.

Those who appear qualified will be given a 90-day work permit and must give the INS their documentation during that period or face deportation.

"Obviously, what we want to do is get qualified people in as quickly as possible," Nelson said.

Growers, especially those in the West, have complained that they face a labor shortage this summer because of difficulty encountered by the Mexican work force in crossing the border.

The INS said the growers have become complacent about having such an inexpensive, illegal work force and should have done more this year to recruit U.S. workers or assist Mexican workers with the legalization process.

Nelson said he found it "incredible" that no western growers applied for a special guest-worker program, which requires housing and transportation for foreign workers.

"The growers have not done what they should have done during the last six months," Nelson said. "There's been some panic, which may or may not be based on reality, some hype. Where all that balances, I don't know. Nobody wants a labor shortage, and nobody wants crops to rot on the ground. On the other hand, we don't want to turn this into a political football."

Meanwhile, members of a congressional conference committee voted last week to add language to the supplemental budget bill to force the INS to speed admissions of alien farm workers.

Under a law signed last November by President Reagan, as many as 4 million illegal aliens who have lived in the United States since before Jan. 1, 1982, are expected to apply for legal status.

The farm workers, most of them Mexicans, are covered under a separate legalization program that started June 1 and extends for 18 months.

Nelson said only 10,300 applications under the agricultural program were filed in the first 3 1/2 weeks by farm workers who stayed in the United States illegally.

Of those who returned to Mexico after the harvest, the U.S. Embassy reported that only 4,000 had requested applications by last Wednesday.