Spouses of nursing home residents who receive Medicaid benefits could be charged part of their income to help pay the costs under a package of Medicaid changes that the administration is expected to send to Congress soon.

Under the proposal, first reported in the trade publication "Medicine and Health," a state could require a spouse to pay 20 percent of the income he earns above a level equal to twice the government's official poverty line. At present, Medicaid picks up the whole bill because the spouse's income is not counted as income available to the institutionalized person.

The same rule would apply to the parents of a child institutionalized at Medicaid's expense. The poverty line for a single person -- the figure that would be applied in the spouse's case -- is now $5,250, according to Health and Human Services Department guidelines.

For the first time, states would also be able to include people in the program who are poor but who have no minor dependent children and who are not aged, blind or disabled.

The proposed changes for the huge federal-state program, which provides medical care for about 22 million low-income people, would sharply relax federal restrictions on how states can run their programs, moving Medicaid closer to the "block-grant" concept that has long been favored by many in the Reagan administration.

The theory is that if the administration succeeds in limiting the growth of the program, reducing federal payments to the states by up to $6 billion over the next three years, the states should be given more flexibility to run their programs.

This flexibility would permit them to expand their programs, but they will also be able to restrict coverage to save money.

Rep. Henry A. Waxman (D-Calif.), chairman of the House subcommittee with jurisdiction over Medicaid, said the "so-called flexibility package is really a set of tools designed to let the states ration care and shift costs onto the medically needy" because the proposed cap won't leave enough money "for a decent Medicaid program." Waxman said the package "eliminates very basic federal protection for poor children, needy pregnant women and nursing home residents."

Two categories of beneficiaries -- who represent three-fourths of the total -- would not be affected by most of the changes: recipients of cash welfare payments under the federal Supplemental Security Income program and those in the federal-state Aid to Families with Dependent Children program.

Under the proposal, states would still be required to provide all of these beneficiaries with a minimum package of benefits. The beneficiaries also would still be free to choose where to receive their care, and they could not be required to pay more than a "nominal" copayment for basic services.

However, the administration's proposal would, for the first time, permit the states to charge pregnant women and children covered by AFDC some amount for basic services.

The flexibility proposals would be applied primarily to "medically needy" people -- those who have dependent children or are aged, blind or disabled, but are not quite poor enough to qualify for AFDC or SSI -- and persons brought in under the new eligibility rules.

Theoretically, the proposal amounts to a major relaxation of eligibility restrictions that liberals and Medicaid supporters have long supported.

Since the Medicaid program was created in 1965, federal law has limited eligibility to low-income people who are caring for dependent children or who are aged, blind or disabled.

If the states decide to interpret the rules broadly, many millions of people could be added to the Medicaid rolls.