Japanese-American shipping tycoon Kay Sugahara made a formal proposal yesterday to the National Governors' Association for a $10 billion "Partnership for Prosperity Fund," through which the Japanese private sector would put up $10 billion for low-interest loans and equity investments to create jobs in all 50 states.

But beyond that, he was no more forthcoming than before on the exact amount, nor did he say which Japanese companies and banks would advance the money.

"The heavy sugar is what most of us are interested in, whether it's $5, $10, or $15 billion," he said. He suggested repeatedly that with his connections in Japan, he could deliver the money for projects proposed by the governors and approved by "the power structure" in Japan.

Sugahara asked the governors to have project proposals ready by March 25. They will be sent first to the U.S.-Asia Institute here, a nonprofit group formed by Sugahara to promote ties between the two regions.

"The USAI will then take these proposals to Japan for discussions with the power structure," he said. "That way, nobody loses face, and in Japan that is important. For example, if New Jersey says it wants 20 casinos, I don't think the Japanese would like that very much. But they would like things like expanded coal ports very much."

To a large audience attracted by advance word of his proposal, Sugahara stressed that "despite my Asian face," he was appearing as an American, concerned over the nation's declining health, who felt that Japan as a rich ally should help this country "reactivate its economic engine."

Perhaps in recognition that over the weekend the Japanese government had issued public declaimers of knowledge of the proposal, Sugahara said that "whether or not this idea goes through," he had a commitment from 25 leading Japanese corporations to increase their imports to reduce the Japanese trade surplus.

An aide said Sugahara was not surprised at official reaction from Tokyo. "He's good at keeping their names out of the papers," the aide said.

Sugahara told the governors that as Japan's trade surplus grew rapidly last year--it is now estimated at $18 billion for 1981--U.S. Trade Ambassador William Brock "went to Japan and kept their feet to the fire. So we Sugahara and colleagues told the Japanese, 'This is serious--you've got to do something.'

"But the choices they had were not very good. Either buy 10 atom bombs at $1 billion each or cut your exports by 20 percent." Apparently referring to the $10 billion worth of atom bombs, Sugahara said: "Of course I am speaking metaphorically."

It was at that point, Sugahara told the governors, that he proposed his $10 billion fund as an alternative, and he said that"key" politicians and bureaucrats "preferred it 1,000 percent to buying arms or cutting exports." He selected the $10 billion figure because that represents 50 percent to 60 percent of the Japanese trade surplus, "and if somebody pressed them, they'd have to do at least that much" to reduce the imbalance.

Brock told The Washington Post that the plan as previously described--as a "reverse Marshall Plan"--couldn't work. "But if it's a serious effort to get Japanese capital investments into this country, we would encourage anything along those lines," Brock said.