American plans to build outer space defense systems are likely to disturb Western European nations that have depended on nuclear weapons to deter a Soviet conventional attack for the past four decades, the departing French ambassador to the United States believes.

"We Europeans do not believe in conventional deterrence," Bernard Vernier-Palliez said in an interview marking the end of a three-year tour as France's ambassdor here. "Twenty centuries of history have taught us that conventional deterrence does not work . . . . It would be very dangerous if this country lost its belief in nuclear deterrence."

Vernier-Palliez's comments, made in response to a question concerning signficant problems that may crop up in the near future between the United States and Europe, placed on the record what had been a deepening but private European concern about the Reagan administation's still tentative "Star Wars" proposal for a space-based system that would intercept and destroy Soviet nuclear weapons.

The effect of such a system, European officials fear, would be to establish a defensive umbrella over the United States and eliminate any likelihood that the United States would respond with nuclear weapons to a conventional assault on Western Europe by Warsaw Pact forces. It would also greatly reduce the effectiveness of the British and French nuclear arsenals, in this view.

Despite the concern about the future of U.S. nuclear policy, the 66-year-old French businessman-turned-diplomat gave the Reagan administration comparatively high marks for its foreign policy accomplishments, saying at one point that "French-American relations have never been better in decades."

Such a judgment was hardly predictable when Mitterrand named Vernier-Palliez to represent his Socialist government in Washington after the Socialists' 1981 victory over the center-right parties that had led France since 1958.

The ambassador, who had been head of France's Renault automobile firm and who had visited the United States frequently to oversee Renault's investment in American Motors Corp., suggested in the interview that he had initially shared some of the questions that Reagan administration officials voiced in the wake of the leftist victory in France.

"After all, I was not a Socialist. I was not involved in politics, and most of my friends were not Socialists . . . so I asked for definitions, and commitments, on what the foreign policy would be, and I was satisfied. And the commitments have been kept."

Mitterrand's early decisions to carry out pledges to nationalize some of France's major industries and to include four Communists in his Cabinet upset Washington at first. But relations quickly stabilized and then improved dramatically after Mitterrand voiced strong support for the Reagan administration's nuclear rearmament program.

"In such a situation, you must be honest. You must interpret the United States to people back home, and explain France to this country, and you must explain the bright points and bad points when the time comes. But mostly you must explain 'why.' That is what we have done over the past three years, and I must say that French-American relations have never been better in decades," Vernier-Palliez said.

He leaves Washington with a very different idea of the United States than he had formed during his career at Renault. Traveling with his wife as ordinary tourists across the United States, Vernier-Palliez has taken his summer vacations here rather than at home, and, he said, discovered "Main Street" in the process.

He was impressed by the openness of American citizens, but also concerned about his finding that "on the whole, the United States is very little aware of the impact of its economy on the world economy. This year, the United States is likely to have an $80 billion balance-of-payments deficit following a $45 billion deficit last year, and below the expert level nobody worries, because nothing happens" outwardly to the economy.

"The United States faces virtually no constraints, like those faced by a country like France, on its economy," he continued. "The United States position is thus much more comfortable. But it is also more dangerous, because constraints operate as safeguards."

He praised the "entreprenreurial spirit" that had helped bring the United States out of a crisis of pessimism at the beginning of his tour.

"What France can take as a lesson is this entrepreneurial spirit, and the flexibility of this economy. Here you have the right to fail. In France,. . .if you went bankrupt, you dishonored not only yourself but your sons and your grandsons. So we were not allowed the right to . . . fail."

Vernier-Palliez introduced his reservations about the outlines of the Reagan administration's Strategic Defense Initiative, popularly known as Star Wars, by noting that on nuclear matters up until now, Paris and Washington "have seen eye to eye."

He then voiced his concern about any steps that would weaken belief in nuclear deterrence. "Heads of state are much more cautious if there is any risk of nuclear escalation of conflict . . . All the French people believe in their own defense, and whether you like or not, nuclear defense has been effective. It would be very dangerous if the United States lost its belief in nuclear deterrence. A defensive system that would make offensive systems useless, as has been suggested, is something that would concern us."