Japanese trading companies are considering the purchase of up to $300 million in American and European commercial jet planes that they do not need, and leasing them at favorable rates to financially pressed British and Southeast Asian airlines.

Japanese atomic power firms are discussing a plan to pay American companies up to $2 billion now for enriched uranium that this country will not need for another seven or eight years.

Despite appearances, such unusual plans are not signs that the Japanese have become easy marks for a sales pitch or are losing their grip. As explained by senior government officials in interviews here, the aricraft and uranium deals are among the active proposals for billions of dollars in "emergency purchases" intended to reduce this country's embarrassingly high - and still soaring - surplus in balance of payments.

The Japanese, who feared disaster at the time of the 1973 Middle East oil embargo and OPEC price explosion, have adjusted to the new facts of international economic life more easily than they had imagined they could. They have adjusted so much better than the rest of the world in fact, that the United States and European countries are insisting on strong action to correct the huge excess of Japanese exports over imports.

Last year's balance of payments surplus of $14 billion was deemed unacceptable by other advanced nations, which suffered economic strains including unemployment because of big domestic sales of competitively priced, high-quality Japanese cars, television sets, steel, optical equipment and other products. Tokyo was told to sell less and buy more or face severe retaliatory action.

Early this year Japan agreed to a long list of tariff reductions and other import-easing measures, and on unofficial target of $6 billion was established for the payments surplus for the year.

Nevertheless, Japanese officials indicated in interviews that the payments surplus is still running at or above last year's level. One senior official, who asked that he not be identified, said that up to $4 billion in "emergency purchases" may be made to bring the surplus down to $10 billion or so by the end of the year.

This is an unexpectedly dismal result for Prime Minister Takeo Fukuda to have to report to President Carter and European leaders at the Bonn economic summit July 16-17. Calling the payments surplus his most serious problem, Fukuda said in an interview that he has taken measures to freeze the quantity of major Japanese items sold abroad. But he expressed frustration that the dollar value of sales is still rising because of inflation abroad and the sharp decline of the dollar against the yen on foreign exchange markets.

"Japan will do its best, and we will take any measures that we can do," said Fukuda. But he said the dollar value of Japan's exports is "up to the United States" and gently chided the United States for failing to control inflation and curb the oil imports that contribute to the weakness of the dollar.

Fukuda said is "very difficult" - often a polite Japanese way of saying no - for him to accede to U.S. demands to permit more citrus fruit and meat into this country. Farmers are a major part of the ruling party's constituency, and they have been strongly resisting dismantling of the import barriers that protect them.

Fukuda alluded to the plan for emergency import measures, but gave no details. Other officials said the proposed purchase of commercial jetliners, which is being promoted by the government, could bring "rent-a-plane" arrangements between Japan and Britain, South Korea, Indonesia, the Philippines and Thailand.

The purchase of the planes would be financed by Japanese banks at low interest rates, permitting lease arrangements on favorable terms by countries whose airlines need new planes but lack the capital to buy them.

Another plan being discussed in the Japanese Cabinet is the purchase of several special-mission airplanes for the use of the prime minister and other high government officials. Japan, which has a minimal air force for a nation its size and is barred by its constitution from sending military forces abroad, sends its officials around the country and overseas on commercial planes.

In view of the need to spend money quickly abroad to balance this country's economic overachievement, "Rising Sun One," operated by civilian pilots and crew, may soon take its place on runways near the sites of conferences of world leaders.