The Reagan administration retreated yesterday on Social Security, saying the entire list of benefit cuts it proposed only a week ago is "negotiable," including the very large cut in the benefits of those who will retire in the future before age 65.

"We have 13 provisions in this bill and they're all negotiable. We're certainly reasonable men," Secretary of Health and Human Services Richard S. Schweiker said as he sought to reassure the House Select Committee on Aging after members of both parties had indignantly denounced the president's recommendations as excessive and unfair.

His appearance came a day after the Senate, led by its Republican majority, went on record 96 to 0 against either "precipitously and unfairly reducing benefits of early retirees or cutting benefits more than necessary to maintain the solvency of Social Security. Partly to help reduce budget deficits, Reagan proposed cutting nearly twice that amount.

In a further response to that vote, President Reagan sent a conciliatory letter to congressional leaders late yesterday, emphasizing that his only purpose was to save Social Security from "bankruptcy" and saying he would willingly back off from his proposals in favor of any "bipartisan" alternative that would do the job.

Schweiker said the administration would even discuss reductions in future cost-of-living increases for Social Security recipients, which some in Congress have advocated as a fairer way of shoring up the system in the future, but which the president up to now has opposed.

The stutter-step on Social Security came as the administration also continued negotiations with Democrats and skeptical Republicans over the president's tax cut proposals, which also are in some trouble. (Details on Page D1)

The proposed Social Security cuts, which Schweiker helped put together and the president approved, miscalculation of the Reagan presidency.Democrats have seized on them to throw the Republicans on the defensive and reunite their own party, badly split by Reagan's earlier budget-cutting efforts on Capitol Hill.

Reagan proposed cutting benefits generally for those who will go on the rolls in the future; cutting them especially for any who retire before age 65 as an inducement to workers to stay on the job longer, and putting important new restrictions on future disability benefits.

Even though the administration said they were necessary to maintain Social Security's future integrity and might permit future tax cuts, they were instantly criticized. Yesterday the criticism continued, mainly but not solely from Democrats.

Chairman Claude Pepper (D-Fla.), opening the Committee on Aging hearing, said, "The impunity with which this administration goes about slashing promised benefits is both astounding and frightening." Rep. Mario Biaggi (D-N.Y.) said the cuts would be "a blatant assault on the entire system."" Rep. Tom Lantos (D-Calif.) called the proposals "appalling." Rep. Olympia Snowe (R-Maine), without making any direct criticism, said she wondered; "if the White House switchboard has been ringing like the phone in my direct office."

On the other side of the Capitol, Sen. Edward M. Kennedy (D-Mass.), holding a hearing of opposition, called the administration proposals a "severe and drastic retreat" from government commitments to income security for older people.

Many Republicans defended Schweiker as having forced the system's problems to public attention but even they showed hesitancy over the specific proposals. House Minority Leader Robert H. Michel (R-Ill.) was quoted by a wire service as saying he could not support cuts that would jeopardize workers approaching 62 who had planned to retire under the existing provision providing only a 20 percent permanent cut at 62.

Schweiker, while agreeing to compromise, said it would be a "shell game" to think that Social Security's financing difficulties can be solved easily. "There are no pleasant alternatives," he declared, only difficult and painful ones.

While House deputy press secretary Larry Speakes said, "We think (the president's proposal) is the best plan yet offered. We ask 'What is the alternative?'"

Senate Republican leaders and members of the Finance Committee, which has jurisdiction over Social Security, said in interviews that they had not been consulted in advance on the Reagan proposals. If they had, they said, they would have warned Schweiker, a former senator, and the White House that, as Finance Chairman Robert J. Dole (R-Kan.) said, "It wouldn't fly."

Dole said Social Security's immediate problems can be met by interfund borrowing between the old-age fund, which is near depletion, and the disability and Medicare funds, which have plenty of money right now, and "we'll look at the cost-of-living adjustment" for a possible cut.