The Reagan administration, encouraged by a softening of opposition from congressional liberals, is preparing to approve the long-delayed sale of U.S.-made helicopter spare parts to the military government of Gen. Efrain Rios Montt in Guatemala.

A State Department spokeswoman denied that a decision had been made. However, there is a consensus among department and White House policy makers that the sale should be made, and reliable sources said yesterday that a formal go-ahead by President Reagan and Secretary of State George P. Shultz is expected imminently.

In addition, Rep. Michael D. Barnes (D-Md.), chairman of the House inter-American affairs subcommittee, said yesterday that based on his talks with administration officials, "I think there's absolutely no doubt that the administration is going to do it."

Barnes spearheaded the congressional opposition that has forced the administration to delay the sale for 18 months. However, he and others in Congress who had criticized the move on human rights grounds have modified their opposition to the point where the administration feels it can proceed without provoking a major controversy on Capitol Hill about its policies toward Central America.

In a telephone interview yesterday, Barnes denied that he had given his blessing to the sale, saying, "I'm still looking at it." However, he also noted that a recent investigation by committee staffers of Guatemala's human rights situation indicated "that things are very different there now--that there are substantial changes in the mood of the country about the regime and about the way the Army operates."

"I don't think it would be the most outrageous thing in the world to sell them these parts," he added.

At issue is Guatemala's longstanding desire to buy $2 million to $3 million worth of spares for 11 helicopters used to fight a growing guerrilla insurgency. The Reagan administration, fearing a leftist takeover in Central America's most populous country, has been seeking ways to bolster its ties with Rios Montt, who took power after a coup last spring.

In recent weeks, the administration has intensified its efforts to win backing for the sale on the grounds that Rios Montt is in danger of being ousted by hard-line elements within Guatemala's military and needs a show of support from the United States.

During his visit to Latin America last month, Reagan met with Rios Montt in Honduras and told reporters afterward that the general "is totally dedicated to democracy" and is getting a "bum rap" from critics in this country.

Because the proposed transaction involves a cash sale of equipment not designated specifically as having a military purpose, the administration can give it a green light without the approval of Congress. But the administration has been deterred from doing that because of widespread congressional complaints that the Guatemalan military has been involved for years in a campaign of murder and other rights abuses against its domestic opponents.

Because of the human rights situation, direct U.S. military aid to Guatemala has been cut off since 1977. At that time, Guatemala refused to apply for aid because of a critical State Department report on the rights situation there.

Congress steadfastly has refused to grant administration requests to resume the relationship through small appropriations for training.

In July 1981, following discussions with Thomas O. Enders, assistant secretary of state for inter-American affairs, Barnes and Rep. Stephen J. Solarz (D-N.Y.) agreed to withdraw legislation that would ban military aid to Guatemala.

The two congressmen and their allies have contended that, in return, Enders promised the administration would not permit the helicopter spare parts sale while the congressional objections remained. Enders and other administration officials, while acknowledging they had agreed to consult fully on the matter, deny that they gave Congress an effective veto over the sale.

Still, the administration, aware of the strong feeling on Capitol Hill, had continued to move cautiously because it feared an adverse reaction not only over Guatemala but also against the significantly larger aid amounts it wants for El Salvador, where support for the government's fight against leftist guerrillas is the linchpin of U.S. policy in the region.

Barnes met with Enders Dec. 23. In addition to Barnes, key House members whose opposition to the sale reportedly has weakened include the chairman of the House Foreign Affairs Committee, Rep. Clement J. Zablocki (D-Wis.).