The Reagan administration is preparing to announce a final decision not to sign the international treaty on Law of the Sea, rejecting pleas that the United States continue to bargain in hopes of making changes.

The decision, made by President Reagan after a National Security Council meeting on the subject June 29, risks eventual U.S. isolation from a new global system accepted by the rest of the world.

Four key allies, which had been pressing for word of Washington's intentions, were informed by the State Department last Friday that consultations scheduled with them Tuesday and yesterday had been postponed in light of a forthcoming announcement.

The allies were told, without specific details, that the announcement would be negative.

An administration source said the postponement was made because of concern that the consultations, coming at the same time or just before the administration's announcement, could have embarrassed Britain, France, West Germany and Japan by seeming to associate them with the U.S. decision.

One of this administration's first acts was to reverse the sea law policies of the Ford and Carter administrations, which had participated in bringing the talks to the edge of completion in difficult negotiations with Third World and industrial nations.

The Reagan administration blocked final action on the document early last year and, after nearly a year of study, decided in January to return to the bargaining table to pursue broad changes, especially in the economically important sections about mining the riches of the sea beds.

After a policy battle within the government, Reagan decided in April that the changes did not go far enough. Thus, on April 30, the United States was one of four nations to vote against the treaty, which was adopted by 130 countries, including France, Japan, most western nations and nearly all of the developing world.

Seventeen nations, including most of the Soviet bloc and a few industrialized western countries, abstained.

In the latest round of controversy, several key allies and U.S. supporters of the treaty inside and outside government urged the administration to continue efforts for changes, in hopes that the document could be accepted by the administration.

A meeting scheduled for September in New York, according to treaty proponents, offers another opportunity for improvements if the United States were to stay in the bargaining.

By announcing now that the United States will not sign the treaty when the signing period begins late this year, the administration is averting a possibly more difficult decision later in case of new international concessions aimed at winning U.S. support.

"We think it is unrealistic to believe that further negotiations can make enough changes" to make it acceptable, an administration source said. "We see no possibility that the change would come close to meeting our overall objectives."

Reagan's decision, the source said bluntly, was to halt U.S. efforts and "get out now" rather than go deeper into the bargaining.

A "technical level" U.S. official is to be sent to a treaty-drafting conference starting Monday in Geneva, but this was described by administration sources as merely a defensive move to protect against changes in translations. The drafting conference is to make an official translation from English to eight other languages.

Reagan's decision was made against the advice of 85 House members, headed by Rep. Clement J. Zablocki (D-Wis.), chairman of the House Foreign Affairs Committee, and Rep. Benjamin A. Gilman (R-N.Y.).

The lawmakers, in a letter June 23, asked the president to continue full U.S. participation in remaining treaty conferences and to delay a final decision on signing the document until after a congressional review.

Writing in the current issue of Foreign Affairs magazine, Washington attorney Leigh S. Ratiner, deputy chairman of the U.S. delegation to last spring's final negotiating session, said it is likely that most major nations will join the sea law treaty even if the United States stays out.

Should this take place, he wrote, "We will suffer a significant, long-term foreign policy setback with grave implications for U.S. influence in global economic and political affairs."

Ratiner also charged that the costs of U.S. isolation could include serious economic disadvantages and a possible challenge to U.S. navigational rights through international straits.

A senior administration official dealing with the issue, however, said there is little or no prospect that the sea-bed mining provisions will become effective with the United States out. He said there is a substantial prospect of working out an alternate sea-bed mining system in cooperation with U.S. allies.