The Reagan administration acknowledged yesterday that one of every six families on welfare would be forced off the rolls or have its benefits reduced under the budget cuts and rules changes the president proposed this week.

The administration's estimates were cited at a House public assistance subcommittee hearing by Chairman Fortney H. (Pete) Stark Jr. (D-Calif.), challenging assertions by Health and Human Services secretary Richard S. Schwieker that the Reagan budget cuts would not hurt "the truly needy."

Schweiker had just finished saying that safety-net" programs for the aged and needy would rise from 37 percent of the federal budget in 1981 to 41 percent in 1984 under the Reagan program, which he said showed his department was "not exactly being short-changed or cut to pieces," when Stark, the Congressional Black Caucus and others began firing away.

Stark said Schweiker's department had produced a table showing that 400,000 families would be pushed off Aid to Families with Dependent Children and another 258,000 would have benefits cut, meaning that a total of 658,000, or about one-sixth of the 3.8 million families on AFDC would be affected.

Virtually all of these people, Stark said, are below the current $8,410 poverty line for a family of four, belying claims by President Reagan and Schweiker that the "truly needy" would be unaffected by the proposal budget cuts.

For similar reasons, Stark and Rep. Shirley Chisholm (D-N.Y.), representing the Congressional Black Caucus, also attacked Reagan's proposal to combine about 40 mostly small health and social service programs into four large block grants to the states with a 25 percent cut in funds but broad discretion on how to use the money.

Chisholm said many of these programs are deliberately focused on the poorest and most disadvantaged groups in society, and that past experience indicated the "probability that funds would be shifted from survival programs into programs that appeal to middle-class voters or local power structures." She predicted a "devastating impact on the poor, the disadvantaged, the elderly."

Another witness, actress Jane Russell, testified on behalf of WAIF, a child-help group, and the Child Welfare League that a program just passed by Congress, allowing welfare money to be used to put foster children in adoptive home, would be undermined totally if the funds were included in a block grant and could be spent by the states for other purposes.

Schweiker conceded that some of the impacts of the proposals could be "painful," but insisted that, without some cuts, the government will never be able to get control of the economy.

Among the AFDC changes outlined by Schweiker yesterday: requiring beneficiaries to work for their benefits in community-labor projects; making beneficiaries who are college students register for work; counting food stamps and federal housing subsidies as income in determining eligibility; counting the income of step-parents; tightening wealth tests so no welfare household could have more than $1,000 in assets aside from a house and car, and creating a national register of recipients to avert multiple applications by one person.

Schweiker got support from Rep. Willis D. Gradison Jr. (R-Ohio), who called some of the proposals "a breath of fresh air," and Rep. John H. Rousselot (R-Calif.), who said, "Nobody is going to be left starving in the streets."