Abol Hassan Bani Sadr, who has emerged as the most powerful layman in Iran under Ayatollah Ruhollah Khomeini, wants to model the Iranian economy on a 16th century Persian kingdom that he says was an Islamic cradle-to-grave welfare state.

Bani-Sadr, a member of the ruling Revolutionary Council, the de facto foreign minister and the economic overlord of the government, is a French-educated academic economist who claims that Islam provides all the economic answers to man's plight that have escaped solution both by Marxism and capitalism.

There hardly seems to be an important topic from the role of women in society to the reorganization of the army on which this voluble cross between a religious mystic and an intellectual politician does not seem to have formed a strong opinion.

His bifocals and Charlie Chaplin mustache seem to give the 46-year-old Bani-Sadr the air of an Iranian Leon Trotsky. He is the most radical of Khomeini's major nonclerical disciples.

Interviewed in April in his spacious, comfortably furnished middle-class apartment in Tehran, Bani-Sadr claimed to have collected all the answers to Iran's problems and to have provided the stragety that overturned the shah's regime.

He brought out a wooden card-file box, saying, "Here is an x-ray of Iranian society. Everything is in here, all the facts. I have many more boxes like this."

With the contents of the boxes, he said, he was able to predict the crisis of the old regime.

"I proposed the apropriate methods of struggle," he said. "Never before in history was a struggle waged on the basis of all the weak points of a regime. I proposed it."

Asked how the boxes had led him to outline the struggle, he said he had crossed-indexed everything in the Moselm holy book, the Koran, about the Organization of society.

"For example, I have all that has been said on the Islamic model of marriage," he said.

Just that week, he had been on television talking to a group of young people. A young woman wearing a traditional Moslem head scarf said that she did not previously cover her hair, but had been told since the revolution that women's hair gives off rays that are harmful to men. She asked whether this was true.

Bani-Sadr answered that women's hair gives them power over men and that if everyone is to be equal, any factor that gives anyone ascendancy over someone else must be eliminated. Ideally, he said, everyone should be dressed alike and the garments of men and women should be indistinguishable.

Bani-Sadr said that Karl Marx had not understood that once economic inequalities are eliminated, other inequalities -- "beauty, age, intelligence" -- crop up.

"We want a society," he said, "where power relations are elimated, a society where the material aspect is not cut off from spirtuality. We will find a way out of the Marxist-capitalist impasse."

He added: "Equality is possible in the movement, but not in the function. In infinity, total equality is reached. It is a dialectical process." He did not explain.

As he spoke, a middle-aged woman in a headscarf padded back and forth in stocking feet serving the small glasses of tea and sugar that are essential to any discussion in Iran. Overhead glistened a large copper art deco chandelier featuring about half a dozen mermaids with prominent breasts.

Bani-Sadr, who spent about 10 years in Paris working on a doctoral thesis he says he never finished because he feared he might then be expelled as a dangerous foreigner, has many of the ways of expressing himself typical of a Left Bank intellictual.

Despite his stress on equality, BaniSadr also seems convinced of the need for a clearly defined central authority. He was one of the most consistent opponents of the government of premier Mehdi Bazargan, saying that the government's weakness, "letting everyone talk at once," had led to a multiplicity of power centers. He called for more consistent censorship.

"The absence of a strong central government and the extension of anarchy will wind up undermining the authority of Imam Khomeini himself," Ban-Sadr recently said. "A country cannot be governed with permanent popular spontaneity."

He refused in July to become Bazargan's deputy economy minister. Now he has the job of economic overlord, with authority over eight ministries including petroleum, agriculture, industry, finance, water and electricity.

His immediate formula for relieving economic paralysis involves the same practical, Western-style solutions that Bazargan advocated -- simply getting as much money as possible circulating rapidly by putting people back to work.

Ultimately, he says, however, "if we follow the imam, society wil return to the era of the Prophet, or at least to the time of the 16th century Safavid Dynasty under which "the life of man qas assured from birth to death."

That Persian dynasty fell only because it was pushed by outside forces, Bani-Sadr said. He added tht the major problem of the new Iran will be to protect itself from external influences if it is to reach of the kind of internal perfection he seeks.

"We must neutralize the international power relations," he said. "Islam is relations without force . . . I don't exclude that there are possibilities of deviation coming from outside. We cannot control all the power relations in the world."

He insisted, however, that he does not want to try to isolate Iran economically, cutting it off from trade with the outside world.

At home, he favors a Gandhian-style village economy. Tehran, he indicated, may meet the same fate as Saigon or even Phnom Penh after the revolution. He told an interviewer last week that the capital would be depopulated so that it ceases to be "the monstrous parasite city that consumes half of everything in the country and weighs execessively on the state budget." Villages will be turned into "units of production" and four out of five bureaucrats will be transferred to "productive activities," presumably in the countryside.

There will be no room in this vision for U.S. investments.

"In the furture there will be no relations with the multinationals . . . Destructive investments and products must be eliminated -- Coca Cola, Pepsi and Canada Dry. There will be more investments from the United States. Such investments did not benefit Iran. The profits left the country."