Bowing to mounting pressure from farm-state legislators, the Reagan administration yesterday outlined a new relief package to help farmers and country banks through a predicted spring credit crisis.

Budget director David A. Stockman and Agriculture Secretary John R. Block spent more than two hours in a closed meeting with key Senate Republicans, laying out details of the package.

Earlier in the week, the White House, which has been moving to cut farm programs, said no such credit program was contemplated.

Although subject to change, the White House offer would:

* Establish a policy of "forbearance" in federal bank regulation. This would mean that country lenders would have more leeway in extending credit to farmers whose assets have declined, largely through deflation in land prices.

* Allow banks to reduce part of the interest on a farmer's loan -- easing his repayment load -- under the federal loan guarantee plan announced last fall by President Reagan. Current policy covers only the loan principal -- not the interest -- although interest is the lion's share of loan payments in early years.

* Put teams of credit specialists to work with rural banks in handling the crush of paperwork for loans that farmers need for spring planting.

According to senators who attended the session, the administration officials agreed to refine some details of the package and return to Capitol Hill for more discussion within a few days.

"We're working on it," was Stockman's only comment after the meeting. He and Block then left to meet with House farm-state Republicans during the evening.

The sudden appearance of Stockman and Block on the Hill followed days of intense campaigning for credit aid by farm-state legislators, particularly Republican senators who face critical reelection campaigns next year, and after days of administration rejection of their efforts.

"I'm encouraged," said Sen. Mark Andrews (R-N.D.), one of the GOP farm-state senators who has led the call for more help from Washington. "This means they are understanding there is a credit emergency out there."

Andrews said, however, that he and others at the meeting were concerned that the administration proposal "was not as responsive as it ought to be" and that Stockman and Block had agreed to revise it somewhat.

Stockman and Block apparently entered the meeting with a proposal to offer the new credit aid program in exchange for a commitment of support from the senators for the administration's proposals for a "market oriented" 1985 farm bill that has been criticized widely by Republicans and Democrats.

Sen. Jesse Helms (R-N.C.), chairman of the Senate Agriculture Committee, said there was "almost unanimous agreement that existing farm policies and programs are not working . . . that they are a disaster."

But Andrews said he and other GOP colleagues shot down suggestions that there be a tradeoff. "There started to be a quid pro quo, but after 45 minutes they backed away from that. I told them not to be so stupid as to try to do that because we've got a firestorm out there on the prairie," he said.

The pressure for quick help from Washington has mushroomed during the past two weeks as farm-state public officials, farmers and bankers warned that thousands of farmers face foreclosure if they can't get loans by March 1.

Additional pressure and frustration have come from the apparent failure of Reagan's preelection debt-restructuring program, announced on the eve of a campaign trip to the Farm Belt.

Last fall's program included $630 million of additional funds for Farmers Home Administration lending programs, and it provided federal guarantees of loans in which the bank agreed to write off a portion of the principal.

The FmHA assistance has been caught in a tangle of red tape and, according to the agency, only a small percentage of farmers' applications have been approved. Country banks, saying they are too strapped to write off parts of loans, have shown little enthusiasm for the guarantee plan.