The French Socialist Party is expected to win an absolute majority of the National Assembly in runoff voting Sunday, consolidating the leading position it took a week ago in the first round of parliamentary elections.

A final election simulation based on polling Thursday and yesterday projected a total of 275 to 285 seats for the Socialists in the 491-member Assembly, according to IFOP, France's most respected polling organization. The party had 117 seats in the last Assembly.

Sunday's parliamentary runoffs were billed in today's French press as the "final round" in a series of four votes since April 26, when Francois Mitterrand emerged as the favorite to capture the presidency. The Mitterand bandwagon has picked up more steam in every successive round since then.

IFOP predicted that the Communist Party would get 35 to 38 seats Sunday, compared to 86 in the last legislative elections in 1978, the Gaullists 80 to 90, compared to 155, and the Giscardists 70 to 85, compared to 133.

Under French electoral regulations, the polling results could not be made public in France. IFOP misjudged the April 26 first round of the presidential election, but has revised its methods and has been issuing consistently accurate confidential polls since then.

In the first round of the election last Sunday, 156 deputies received majorities and were elected outright. One hundred of those were Gaullists or Giscardists since they had agreed to support each other's incumbent deputies from the start. But they were left clearly leading in only 32 races.

The Socialists, who elected 48 deputies last Sunday, compared to none in the first round of the 1978 elections, had substantial leads in 214 runoffs. The Communists won only seven seats the first time around and were leading in about 30.

The Socialists reportedly had been prepared to withdraw front-running candidates to let perhaps a half-dozen Communist candidates, including several top national leaders, keep their seats, But, Socialist sources say, the Communists asked for no special favors in the brief negotiating session and the two parties simply confirmed a previous agreement that candidates of the two main leftist parties would withdraw in favor of each others' front-runners.

The Communists are said to have feared that even if the Socialist leadership made suh a deal, it would be disavowed by the triumphant Socialist electorate, swollen by disaffected Giscardists and other center-left anticommunists, creating the possibility of still another humiliation that the Communists would prefer to avoid.

The Socialists were nevertheless understood to be prepared to try to save the seats of second- ranking Communist Charles Fiterman and of Paris party chief Paul Laurent, considered the head of the "liberal" wing. Of the 11 members of the Communist Party's 21-member Politburo who had parliamentary seats, six have been eliminated from the National Assembly.

The Communist debacle changes the French political landscape even more profoundly than the Socialist sweep. The question is still open whether the Communists will enter Mitterrand's Cabinet, but the nature of the Socialist triumph seems to have totally transformed the meaning of Communist membership in the government, reducing it to a form of political symbolism that obviously no longer frightens moderate voters despite efforts by Gaullist and Giscardist campaigners this week to revive the Communist specter as an electoral argument.

The French voters give the overwhelming imression that they have accepted as the logic of the constitution designed by Charles de Gaulle for himself that the president of the republic be given a majority of supporters in the National Assembly, whether he is of the left or right.

The gaullist authors of the constitution clearly intended to ensure continual electoral victories of the right out of fear of heavy Communist influence if the left ever won. But that rationale has been altered by the collapse of the Communists in successive presidential and legislative elections this spring.