RE-ENCOUNTERS IN CHINA; Notes of a Journey in a Time Capsule. By Harold I. Isaacs. M.E. Sharpe, 80 Business Drive, Armonk, N.Y. 10603. 192 pp. $19.95.

WHILE EVERYONE goes to China now -- including one's dentist and writers for The New Yorker -- I cannot imagine that any other account by an American of two weeks in Peking and Shanghai in October 1980 could have yielded fruits as rich as this memoir by Harold Isaacs.

In the early 1930s, when he was in his early twenties, Isaacs edited in Shanghai a Communist-sponsored newspaper, the China Forum. He came to know many of the leading leftist Chinese intellectuals who were young too in these years when they confronted the repression of the Kuomintang regime. The China Forum collapsed and Isaacs was abandoned by his Communist friends when his own political outlook took him in a Trotskyist direction. In Peking, where he lived during 1934-1935, he wrote a classic account of events in China in 1925-1927, The Tragedy of the Chinese Revolution (still in print in a revised edition first published in 1951), which made it impossible ever after for any serious scholar to heed Stalin's version of the portentous episode of the "first united front" between the Communist Party and the Kuomintang.

In succeeding decades, as a liberal intellectual now without the burden of Trotskyist chimeras, Isaacs was Newsweek's war correspondent in China until expelled in 1945, reported on immediate postwar development in Asia, and then in the early 1950s began a distinguished career as a professor of political science at MIT, from which he retired in 1976. Apart from authoring half a dozen important books on cultural confrontations and the impact of political change on group identities, in 1974 Isaacs published Straw Sandals: Chinese Stories 1918-1933, which he had first undertaken in Shanghai in the early 1930s with the advice and assistance of the famous writers Lu Hsun and Mao Tun.

BUT from the demise of the China Forum until the enormous changes that followed the death of Mao Tse-tung, Isaacs was a non-person in China -- his account of "The Doctored Photo" in this memoir is an instructive example of the Leninist proclivity to rewrite history. Even the redoubtable Soong Ching- ling, Sun Yat-sen's widow and vice chairman of the People's Republic -- "Suzie" to Isaacs in Shanghai in the 1930s -- was unable to invite this old friend on her own. Isaacs, and his wife Viola who had shared all these experiences, came at last to revisit China as guests of the Chinese Writers Association in 1980 largely because of their earlier association with Lu Hsun, whose letters to Isaacs about the compilation of Straw Sandals had found their way to the People's Republic.

The body of this evocative and supplely written memoir consists of Isaacs' notes of his conversations with these friends of his youth: with Soong Ching-ling, nearly 90; Mao Tun, 84 -- both died in 1981; the famous woman writer Ting Ling; the economist Chen Han-seng, and others. Many had spent 20 years in internal exile or imprisonment, victims of Mao's anti-rightist campaign of the 1950s and the Cultural Revolution of the 1960s and 1970s. The vignettes are brief and by the choice of those interviewed, the accounts of their experiences and current outlooks are not always very revealing. But there is enough to rivet the reader's attention, aided by Isaacs' double exposure technique of juxtaposing both ends of half a century.

MOST OF the conversations center on personal experiences, with the broader political scene refracted through these individual lenses. In a concluding chapter, "Of the Larger Politics," Isaacs brings together hisends' and his own observations about such matters as "judging Mao" and "the limits" to political and artistic expression in China today. There have been changes and perhaps some progress since 1980, but these remain yet exceedingly touchy matters.

For the historian of modern China, the pages devoted to Soong Ching-ling are probably the most valuable. She emerges here not as the elderly, smiling plaster doll evoked by China Reconstructs and the like, but as a tough, politically committed, often disappointed, but nevertheless warm and genuinely idealistic survivor. Isaacs sadly notes that he received no positive responses to his suggestions that these prominent personages write their own stories. And for many whom he reencountered in 1980, it was as if he had not lived at all in the intervening five decades. For a few brief hours the threads sundered in the 1930s were picked up again.