Charles Hernu, defense minister in France's Socialist government, has voiced praise for the Reagan administration's defense spending programs and has warned critics of large military budgets against spreading "panic in the public opinion" of western democracies.

Such spending should be accompanied by "clear and vigorous explanations to our national legislatures and to public opinion" to combat the growth of neutralist sentiment not only in Western Europe but also the United States, Hernu said in an interview here Tuesday following a day of annual talks with Secretary of Defense Caspar W. Weinberger at the Pentagon.

"There is, after all, the American episcopate that sends letters to French bishops and prelates that are perhaps more demoralizing than are the [neutralist] movements in West Germany," Hernu said in an apparent reference to a draft pastoral letter drawn up by Roman Catholic bishops in the United States and circulated to French Catholic bishops.

American and European bishops met at the Vatican this week to discuss the draft, which opposes the use of nuclear weapons, but yesterday issued a statement indicating they had failed to come to an agreement.

The Reagan administration has sharply criticized the action of the American bishops.

Hernu's remarks underscored the increasingly close alignment on defense matters of the conservative Republican government in Washington and the Socialist regime of President Francois Mitterrand, despite Mitterrand's naming four Communists to his Cabinet and his insistence that he will not bring France back into the military structure of the North Atlantic Treaty Organization.

"Secretary Weinberger and I understand each other," Hernu said, adding that the bitter American-European dispute over the construction of a natural gas pipeline from the Soviet Union to Western Europe "did not leave any scars" on U. S.-French relations. "We did not even speak of it."

In the interview, Hernu also made these points:

* France would augment the 1,600-man contingent it has contributed to the multinational force in Lebanon and take on greater security responsibility throughout Lebanon after an Israeli-Syrian-Palestinian withdrawal if the United States and Italy would agree to similiar increases for their contingents. He declined to say whether France would keep its troops in Lebanon if the United States withdrew the 1,800 Marines it has there.

* France will continue its own nuclear weapons modernization program no matter what the outcome of Soviet-American talks in Geneva on intermediate-range nuclear missiles, and totally rejects Soviet proposals to have its independent nuclear force used as a basis for negotiations at those talks. Hernu indicated that he expected there would be significant movement away from the United States' "zero option" opening proposal and the Soviet Union's initial offer of a moratorium on new deployment. It must occur, he said, if a compromise solution is to be found "somewhere between the two opening positions."

* Mitterrand has not yet decided to add an enhanced radiation, or neutron, bomb to the French nuclear arsenal, although development of the weapon has been proceeding.

Hernu strongly denied suggestions that development of the neutron weapon and of a new generation of tactical missiles called the Hades, which would have a range of 200 miles and enable French troops based in West Germany to hit targets in East Europe, represent a shift away from the traditional French nuclear strategy of massive retaliation. Such a shift would make the French atomic force more compatible with NATO's "flexible response" strategy but would be politically controversial in France.

In pulling France out of NATO in 1966, Charles de Gaulle stressed that possession of even a relatively small independent force of nuclear-armed bombers, submarines and intercontinental ballistic missiles would guarantee France's political freedom from the United States and deter an attack by the Soviet Union, which would be hit immediately by an all-out French nuclear response that would badly damage Soviet cities.

Despite deepening balance of payments problems and a budgetary squeeze on the Socialist government, French defense spending will increase in real terms by 3.38 percent in 1983, Hernu said, after a growth of 4.7 last year. He praised the Reagan administration's leadership in achieving a increase "on the order of 8 or 9 per cent after inflation this year, despite economic problems and the discussion in Congress. It is normal that parliamentarians in France or the United States ask questions, but they must realize that these budgets continue to be budgets of security and vigilance."

Asked about political developments in West Germany, Hernu said that the French government thought that the Social Democratic Party, led by Hans Jochen Vogel, has "a 50-50 chance" of upsetting the Christian Democrats led by incumbent Chancellor Helmut Kohl in March 6 elections. Hernu declined to discuss reports that Mitterrand is upset by Vogel's apparent support for the Soviet idea of including French missiles as the subject for bargaining at Geneva as part of a move away from the zero option proposed by the United States. He said that France would "continue to cooperate on defense and other matters with whatever German government emerges from the elections."

Under the zero option, the United States would agree not to deploy 572 Pershing II and cruise missiles in western Europe in return for the destruction of the 600-plus Soviet nuclear missiles currently targeted on Western Europe.