A sudden surge in public approval of the People's Republic of China and continuing very high respect for Japan were reported yesterday as central findings of an intensive nationwide survey of U.S. views toward Asia.

The survey, conducted last July by the Gallup Organization for Potomac Associates, measured the views of a national sample of 1,616 Americans toward various Asian countries and U.S. foreign policy issues in the region. The study is among the most comprehensive ever made of American public attitudes toward Asia.

The turnabout in public attitudes toward China after the normalization of U.S. relations in January 1979 is a "colossal shift" -- among the sharpest and swiftest toward a foreign country ever recorded in poll results, according to William Watts, president of Potomac Associates, a private research group.

From a 52-percent unfavorable rating with the U.S. public in 1977, esteem for China soared to a 70-percent favorable rating in this year's survey. Perhaps equally important, and more specific in its application, is the finding that a clear majority of polled Americans with opinions on the matter (47 percent to 40 percent) now approve helping China build up its military power as a counterweight to the Soviet Union. In 1977, only one person in eight took this view.

Asked if the United States should come to the defense of China if attacked by the Soviet Union, 45 percent of this year's sample said yes; 42 percent said no.

Those interviewed gave a greater degree of approval for American military involvement in defense of China than they did for the defense of South Korea, if it came under attack by North Korea. Despite the presence of 40,000 U.S. troops in Korea and a historic security commitment, a minority of the sample approved U.S. involvement (38 percent to 51 percent). m

A similar minority of the sample (30 percent to 55 percent) said the United States should come to the defense of Thailand if it is attacked by Vietnam. The survey was completed a few days after last summer's incursion by Vietnamese forces into Thai villages at the Thai-Cambodian border.

One of the surprises in the Potomac study was the extremely favorable attitude toward Japan and the Japanese people, an approval that continued to grow despite recent disputes concerning the importation of Japanese automobiles and Japanese hesitancy to join strong U.S. sanctions against Iran.

This year's survey accorded Japan in approval rating of 84 percent, and recorded a list of highly favorable attributes for Japan, its people and products. The poll reported that Australia and Japan are in a class by themselves among Asian countries in the esteem of Americans.

By contrast, North Korea, Vietnam and the Soviet Union were strongly diapproved of by the sample. The public had ambivalent opinions, reflecting a scarcity of information, about many Southeast Asian countries.