French President Francois Mitterrand is prepared to help America's faltering efforts to organize a multinational peacekeeping force for the Sinai Peninsula by contributing troops to that force, a move that marks a striking reversal of France's previous hostility to the U.S.-sponsored, Egyptian-Israeli peace treaty agreed to at Camp David.

The decision of the French Socialist leader, made known by him to a group of five American journalists during a discussion here today, could represent a breakthrough for the efforts of Egypt, Israel and the United States to convince other nations to join the force despite strong pressures from Saudi Arabia and other opponents of Camp David to boycott the peace treaty.

At a 90-minute breakfast, Mitterrand appeared relaxed and in good spirits following a day of meetings with President Reagan and his foreign policy advisers on Sunday during festivities marking the battle of Yorktown. While differences persisted on some points, which he spelled out in detail, Mitterrand projected a sense that the talks had gone well with Reagan, whom the French leader finds to be open and ready to discuss difficult subjects in a friendly way.

Moreover, Mitterrand, who came to power in May, again linked his concepts of East-West relations firmly with those of the Reagan administration and praised Reagan's decision this month on strategic arms in terms more enthusiastic than most American politicians who have spoken out on the MX missile and decision to build the B1 bomber. Mitterrand's praise seemed designed to underscore the French contention that France is America's most reliable European ally in the East-West confrontation.

For Mitterrand, who carefully avoided commenting on specific parts of the strategic package, Reagan's decision will enable the United States to regain a favorable balance in strategic arms against the Soviet Union by 1985, and is a key factor in getting the Soviets into serious arms negotiations. He repeated a sentiment he voiced in an ABC-TV interview Sunday that the United States must move quickly now to get those negotiations started. But in this longer exposition of his thoughts, it was clear that he agrees with Washington that negotiations have to be conducted from a position of perceived strength.

Mitterrand spoke in French and asked that his remarks be paraphrased rather than quoted directly in English-language accounts of the meeting. On other topics, the French leader:

Indicated that he does not expect a confrontation with Reagan at the Cancun summit of industrialized and developing nations this week about their open differences on aid to the Third World. France will back changes and adaptations to existing international institutions but does not seek the establishment of new world bodies for economic negotiations. He said the summit would be a success if it opened the way for specific steps to regulate world market prices paid to developing countries exporting raw materials, and contributed to a global energy policy.

Declined to mute the disagreement between Washington and Paris over El Salvador, which he said resulted from different analyses of the nature of revolutionary movements in Central America. He sees the insurgent forces in El Salvador not as committed Marxists-Leninists stirred up by outside forces, but as indigenous groups that have turned to warfare to overthrow archaic and authoritarian governments supported by the West.

But his specific comments on the Middle East after the death of Egypt's Anwar Sadat brought France's policies in that region much closer to those of the United States and quite far from those of his predecessor, Valery Giscard d'Estaing. Giscard remained on good terms with the Arab opponents of Camp David and refused to consider seriously France's participation in carrying out any parts of the peace agreements.

Under Mitterrand, France is actively discussing such participation and will join a military unit that is a true peacekeeping force to separate the Egyptian and Israeli armies if those two nations want French participation. The French decision is likely to persuade Italy, which is also considering joining the force, to do likewise.

Mitterrand, a close personal friend of many of the Israeli Labor Party leaders, has also moved to distance France from the Middle East declaration that Giscard got the European Common Market to adopt in June 1980. Mitterrand blocked a reaffirmation of that declaration, which said the Palestine Liberation Organization had to be asscociated with Middle East peace efforts, at the Common Market's July summit in Luxembourg.

The French leader feels that the Israeli withdrawal from the Sinai in April, which he and Reagan agreed should proceed on schedule, will mark the final useful step in the Camp David process, which he has praised repeatedly. It is now clear, he feels, that Egypt cannot resolve the Palestinian problem on its own and that no other Arab country will come forward to join the autonomy talks with Israel, especially since Sadat's assassination.

Mitterrand appears to agree with some senior members of the Reagan administration that it is unlikely that there will be any quick progress in Middle East peace efforts in the near future. He praised the eight-point peace plan presented by Crown Prince Fahd of Saudi Arabia precisely because it came at a moment when Camp David was reaching a dead end and represented a positive step by the Saudis that could serve to keep peace efforts alive. France, he said, does not aspire to a mediator's role in the region.