The special House welfare subcommittee voted its first big cut in President Carter's welfare program yesterday, a $2.6 billion reduction in benefits he proposed for working-poor families with more than minimal incomes.

These benefits would be in the form of special "earned-income tax credits" or tax cuts intended partly to preserve the incentive of those at lower wage levels to keep working.

Carter proposed that, for a family of four, these credits rise to a maximum of $655 a year. In addition, though a family's credit would decline as its income rose. Carter proposed that some credit be given to families of four with income as high as $15,650 a year.

Partly because, as Rep. Joseph L. Fisher (D-Va.) put it yesterday, members were having "a problem explaining to folks" why benefits should go to those with incomes this high, the subcommittee voted reductions. The new maximum would be $504 for a family of four, the new income cutoff point, $12,600.

The subcommittee yesterday also discussed at length, but did not take a vote on, a proposal to guarantee jobs to all heads of household who would be required to seek work under the Carter plan. Carter would provide a fixed number of public jobs - 1.4 million - to those who could not find work in the private sector, but he would not guarantee work for all.

The subcommittee hopes to complete its work on the welfare bill this year. The legislation will then go to three standing House committees next year, then to the House floor, and then to the Senate. Its fate next year is thus uncertain.

Earlier this week the subcommittee killed a portion of the Carter plan that would have stopped benefits from rising with family size for families of more than seven members.

John Kramer, assistant dean of the Georgetown Law School and deputy counsel to the House Agriculture Committee, said that without that restriction, Carter's plan would have allowed "a typical family of 20" to get some tax credit up to an income of $42,350.

Yesterday, however, the subcommittee voted to stick that limit back in the tax credit portion of the plan.

In other votes, the subcommittee:

Authorized those required to work under the new plan to turn down jobs that don't offer equal pay for eqyal work within the same company.

Administration officials endorsed the move, which was not in Carter's plan, saying it would keep those required to work from having to accept minimum wage jobs in a company that pays other workers more to do the same thing.

Refused to exempt from the work requirement the parents of school-age children who don't have access to federally funded day care.

Raised the limit on how much a disabled individual can earn and still be eligible for special disability benefits from $200 a month to $481 a month.

Decided that states should be able to administer portions of the plan if they wish, and that the plan should have timetables written into it to avoid such problems as a six-month wait before those who apply are determined elegible.

The debate over guaranteeing jobs illustrated the difficult political and economic problems the plan faces.

Rep. Augustus F. Hawkins (D-Calif.), who doesn't think a jobs program should be part of welfare plan at all, said that without a guarantee "a work requirement is pure hypocrisy . . . Millions want jobs and can't find them."

Adminstration officials strongly object to the potential costs of a job guarantee. Department of Labor figures indicate each percentage point rise in unemployment could require an extra $830 million to create the needed jobs.

Hawkins is expected to try to remove the entire jobs section from the Carter plan, and consider it next year as part of a general debate on public employment.

If he succeeds, the issue of public service jobs for those on welfare would probably not even emerge from the House Education and Labor Committee until March 1, at the earliest.