President Valery Giscard d'Estaing formed a new Cabinet today in a bid to halt the rapid erosion of his power caused by a year of economic and political setbacks.

Despite the stunning defeats inflicted on his government coalition in municipal elections this months Giscard offered no bold departures in naming the 15-man Cabinet, headed in incumbent Prime Minister Raymond Barre, who also holds the finance and economic portfolios.

Giscard did make some important gestures toward burying his quarrel with his former allies in the Gaullist party. He ousted two senoir ministers who had conducted strong anti-Gaullist campaigns since Giscard came to power.

Interior Minister Michel Poniatowski, Giscard's chief political lieutenant and his closest personal friend in politics, was fired. Giscard had allowed Poniatowski to launch attempts to destroy the traditional Gaullist structures and to form a new government majority based around Giscard.

Centrist leader Jean Lecanuet, who was minister pf planning in the outgoing Cabinet, had joined Poniatowski in those efforts. He also was dropped.

But in the newly fragile political atmosphere here, Giscard's actions appeared to have little chance of stopping the bitter conflict between left and right that is helping undermine the economy.

A year ago, Giscard still had hopes of constructing "an advanced liberal society" through reforms and of boosting France into a position to join West Germany as one of the Common Market's two economically and politically stable countries.

Instead, he has seen his government slide into the ranks of the beleaguered regimes of France's smaller neighbors, and he tactly buried his reform program earlier this week by saying that the primary task confronting the new Cabinet was to stop the leftist alliance of Socialists and Communist from coming to power in the National Assembly election next March.

With much investment paralyzed because of the threat of a victory by the left in the legislative elections and unemployment hovering at the politically damaging mark of 1 million, Giscard and Barre have said they will concentrate on economic recovery as the key element of their 12-month political survival campaign.

A three-month price freeze through the end of December temporarily reduced France's double-digit inflation, but figures for February showed it again rising to above 10 per cent on an annual basis. Barre had suggested that he will take new austerity measures but he has given no details, and political analysts are skeptical that harsh steps will be taken during the uncertain preelectoral period.

Giscard also said that victory would require an end to the divisive between his Republican Independent Party and its Centrist allied on the one hand and the Gaullists, led by former Prime Minister Jasques Chiras, on the other.

Much of the fight, which gave an important boost to the leftist alliance, has been over the role of the Gaullist movement without Gen. Charles de Gaulle to guide it.

In an evident balancing move, the top Gaullist party official in the Cabinet Justice Minister Oliver Guichard, was also dropped.

Replacing him at the Justice Ministry and as the effective No. 2 man in Barre's Cabinet is writer and former diplomat Alain Peyrefitte, who served as DeGaulle's minister of information for four years and who has just written a best-selling critique of French society entitled "The French Illness."

Peyrefitte is a member of the liberal wing of the Gaullist party, but gets along well with the more conservative Chirac.

He left the government after serving as minister of education during the student upheaval of May 1968.

Gaullist sources were also pleased by the choice of Christian Bonnet, the outgoing agriculture minister, to replace Poniatowski at the sensitive Interior Minister, which controls national and local police forces and conducts the elections. Bonnet is a Republican Independent with good ties to the Gaullists.

Peyrefitte and Rene Monory, a Lecanuet Centrist who was appointed minister of industry, were the only new faces in the Cabinet, which was slimmed down from 18 portfolios. Two ministers who were beaten in the municipal elections were dropped, but Michel d'Orano, who lost to Chirac in Paris, was shifted to culture and environment.

Among those staying in place were Foreign Minister Louis de Guiringaud and Defense Minister Yvon Bourges.