An estimated 1 million people are expected to be removed from the Social Security disability rolls over the next two or three years because the law says they don't belong there.

Most of them are not cheaters in the usual sense. They are people who consider themselves disabled, but who no longer meet Social Security's tough eligibility standards. A good number of them will have trouble finding work.

A certain number of people will be removed by mistake and should act fast to find a lawyer. "In this fiscal year we will have to make more than 2 million decisions and some of them will be wrong," says Rhoda Greenberg, director of Social Security's Office of Disability Programs.

Under current law, a disabled person has only 15 days (including mailing time) to tell Social Security that he or she wants to contest the cutoff, and a maximum of 25 days to submit new evidence that he really is unable to work. Many people find that it's hard to get a doctor's report within 25 days, let alone get a case together. If he is turned down again, he has 60 days to appeal to a judge.

In the past, Social Security rarely reviewed disability cases. Once on the rolls, a person normally could expect to stay there. But under a law supported by President Carter and passed in 1980, non-permanent disabilities will be reviewed at least once every three years.

Reviews were not supposed to begin until January, 1982, to give the state agencies time to staff up for the job. But President Reagan accelerated the process. Since March, 1981, 436,000 cases have been examined, and 174,000 people cut off.

The computer scan is starting with younger workers, workers getting higher benefits and workers who entered the program in the mid-1970s when changes in law brought large numbers of people onto the rolls. "Our studies show that people with these characteristics are more likely to be candidates for cessation," Greenberg says. But eventually everyone will be reviewed, including those with permanent disabilities.

Termination comes with startling suddenness. Checks stop roughly two months after a person gets his notice. If he appeals and is eventually returned to the disability rolls (as will happen in many cases), payments will be made for the months he missed. But it could be hard living until then.

A bill now before the House Ways and Means Committee would let payments continue for up to six months while a case is being appealed. (If you lose the appeal, however, you might have to give some of the money back.)

The bill also gives people at least six full months of transition benefits, if they had been on the disability rolls for three years or more; it stops the agency from billing people for back payments if a medical review concludes that their disability ended some time ago; and it gives them up to six months to appeal, instead of only 16 days.

These changes will ease the hardship of sudden and tardy reviews, while still letting them proceed. The practice of billing ineligible people for back payments would resume in 1985.

Regrettably, Social Security beneficiaries get second-class treatment in the government's disability scheme. To be eligible, you must be totally disabled with an ailment expected to last at least a year. You can be eliminated from the program if you appear capable of doing any reasonable work.

A low-income veteran, on the other hand, can get payments for only a partial disability, even if his problem has nothing to do with military service. Federal employes, as well as many state and city workers, enjoy taxpayer-funded disability programs that are far more lenient than Social Security's. Many of the cases of abuse that you read about arise from state or veterans programs, yet only Social Security is up for sweeping review.

Anyone who gets a termination notice from Social Security, yet believes himself to be truly disabled, should probably not risk appealing by himself. Experience shows that you're more likely to get your benefits back if you have a lawyer prepare your case.

To find a lawyer experienced in Social Security work, call a legal-aid office, the Welfare Department or the National Organization of Social Security Claimants' Representatives (toll-free 800-431-2804; in New York state, toll-call 914-735-8812). The fee is usually up to 25 percent of any back benefits you collect, and nothing (except perhaps expenses) if you lose.