Reagan administration officials have informally urged Japan to make far-reaching changes in the structure of its economy, going well beyond reductions in tariffs or trade barriers.

According to a story published June 30 by the Asahi Shimbun, a leading Japanese newspaper, the suggestions were passed on to the Ministry of Foreign Affairs through the U.S. Embassy in Tokyo.

But according to Washington sources who confirmed the essential details of the Asahi Shimbun story, the Ministry of Foreign Affairs was so stunned by the nature of the suggestions that it didn't relay them to Prime Minister Nakasone or to other government bureaus.

The Japanese interpretation of the new American suggestions is that they are tantamount to a demand that the Japanese undergo the biggest cultural transformation in their society since Gen. Douglas MacArthur reshaped Japan after World War II.

Among the "suggestions" are that Japan should:

*Change present laws that attempt to protect "mom-and-pop" stores and facilitate American supermarket-style operations.

*Establish new financial institutions to allow low-interest rate financing of homes, consumer products, and imports generally.

*Reform its taxation system so that there would be a greater incentive for spending money for leisure activities.

*Provide concrete import "targets" going out to the year 2000.

*Restructure industries such as textiles, forest products, and chemicals in a way that would increase imports.

"The United States Government appears to have come to a conclusion that in order to open Japan's market . . . a changing of people's lifestyles is necessary," the Asahi Shimbun story said.

Sources here said the suggestions were in response to a Ministry of Foreign Affairs request for trade deficit-reducing ideas, and that they were coordinated by the U.S. State Department.

A State Department official denied there had been a formal memorandum passed on through the U.S. Embassy. Instead, he said there had been "an informal oral dialogue, listing some concepts that might usefully be addressed in the long run." He stressed "there had been no demands."

Many of the suggestions relate to a belief by American trade and economic officials that Japan needs to change both its working habits and its tax laws in a way that will discourage its present "high savings and low consumption."

The ideas passed along to the Foreign Ministry, according to the Japanese newspaper, were "voluminous," and addressed specifics such as the need for more parks and camping sites in Japan.

The new financial institutions recommended would tap the Japanese postal savings system, offering housing loans as much as 5 percentage points below current rates.

In addition to these ideas, the United States suggested that Japan abolish some of the legal framework now in place that protects depressed industries, and review the role of the agricultural cooperatives that are protecting chemical fertilizers.

The American complaint that Japan prevents the introduction of large-scale supermarket merchandising is an explosive political issue in Japan. Throughout Japan, especially in smaller communities, much of the retail trade is handled by small "mom-and-pop" stores.

U.S. trade negotiators have long complained that the traditional Japanese system acts as a restraint on American mass merchandisers, who want a "one-stop" sales potential. As the system works now, a U.S. retailer must work through many wholesalers, each of whom makes a commission. When the product reaches the shelf, American trade negotiators say, it no longer can sell at a competitive price.