For the second time in less than one year, the Soviet Union has abruptly canceled the visa of a leading American publisher without explanation.

Winthrop Knowlton, chairman of Harper & Row, was informed last Thursday, the day before leaving for Sweden and the Soviet Union, that his visa had been canceled.

Although no reason was given, Knowlton heads the Helsinki Watch Subcommittee charged with dealing with problems impeding the flow of ideas and people between the United States and the Soviet Union. That subcommittee recently commissioned a study that apparently outraged Soviet publishing officials.

The study, by Maurice Friedberg, head of the Slavic languages department at the University of Illinois, acknowledges a pet Soviet point -- that many translated American books are widely available in the Soviet Union but few officially approved Soviet books are easy to find in the United States -- but attributes this to the lack of quality of most approved Soviet literature.

Boris Pankin, chairman of the Soviet copyright agency, attacked Friedberg in the newspaper Sovetskaya Rossiya earlier this month.

He also denounced Knowlton for commissioning the report and criticized Knowlton and Robert L. Bernstein, chairman of Random House, for their roles in Soviet-American publishing relations.

Last August the Soviet Union yanked Bernstein's visa on the eve of his departure for the second Moscow Book Fair.

Bernstein, chairman of Helsinki Watch, has been active in defense of human rights and criticism of Soviet rights abuses.

Even before Knowlton's visa was revoked, Pankin had expressed official Soviet unhappiness by refusing to see Knowlton during the week the American planned to spend in Moscow, according to Knowlton's associates.

Pankin's Sovetskaya Rossiya article says that Knowlton and Bernstein are isolated among American publishers, and names MacMillan, Time-Mirror and Prentice-Hall, "among others," as American publishers whose behavior he approves of.

Knowlton "didn't head for the exhibits" when he attended the first Moscow Book Fair in 1977, Pankin wrote. The implication was that Knowlton used his time to visit with Soviet authors in disfavor with Soviet officialdom.

Knowlton is in Stockholm on the first leg of his planned trip. On Sunday night he delivered a speech on the topic "The Freedom to Publish."