The Carter administration has quietly reversed itself and decided to approve the sale of a large American-made computer to Tass, the Soviet news agency officials confirmed yesterday.

The sale of a similar computer to Tass was held up leas year as part of the administration's protest against harsh sentences given to Soviet dissidents, including Anatoly Scharansky and Alexander Ginzburg.

Two Democratic senators, Henry M. Jackson (Wash.) and Daniel Patrick Moynihan (N.Y.), yesterday lambasted the administration's reversal.

Administration officials said President Carter personnally approved the sale. They said the computer, a Sperry-Univac machine, has been modified since the sale was originally proposed.

Whether Sperry-Univac can sell the computer to Tass is an open question. Last week, a Sperry-Univac official said yesterday, the Soviets announced that they had arranged to buy a computer for Tass from a French firm, CIT-Honeywell-bull.

"We have the [export] license," the Sperry-Univac official said. "We don't know if we have the sale."

In carefully worded answers to a reporter's questions, a senior administration official said last night that modifications to the computer could not be described because such information must remin a trade secret under the Export Control Act.

An aide to Jackson dismissed the modifications as "trival."

The administration official said the decision to sell the computer was based "primarily" on a finding that national security would not be compromised rather than on the basis of foreign-policy considerations.

The Sperry-Univac sale was one of two to the Soviets held up last summer when emotions ran high over trials of Soviet dissidents, particularly the much-publicized Scharansky trial. The other sale involved equipment for a plant to manufacture sophisticated oil drill bits in the Soviet Union.

The drill bit sale was allowed to go through last year.

Jackson and Moynihan contended that Tass, which says it wants the computer to help with coverage of the 1980 Olympics, has "close ties to the Soviet secret police," the KGB.