MOSCOW (AP) _ A Soviet man who got a letter from President Reagan this week about his efforts to join his American wife, has been sent to a labor camp for two weeks on a conviction of ''petty hooliganism,'' his mother said Friday.

Sergei Petrov, 35, a free-lance photographer who has been barred since 1981 from joining his wife in Virginia, was arrested on Thursday outside the U.S. Embassy and was immediately sentenced, his mother, Klavdia Petrova, told a western reporter.

She said a Moscow District Court official told her Petrov was sent to the Beriozka camp near the capital.

Witnesses said Petrov was detained as he tried to enter the U.S. Embassy for an appointment with a consular officer. They said two uniformed Soviet guards led him toward a police booth.

Several months ago, Petrov wrote Reagan asking for help in resolving the cases of about 20 Soviet men and Women who have long been denied permission to join their American spouses.

Petrov received a reply early this week in which Reagan said he found their plight deeply moving and promised to do what he could to help. Petrov was believed to be carrying the letter when he was detained.

Petrov married Virginia Hurt Johnson, 25, a law student from Roanoke, Va., in February 1981 while she was studying Russian at the Pushkin Institute here.

He was denied permission to leave the country with her because officials said he had access to state secrets when he was briefly employed at a research institute in 1976.

In the summer of 1982, Petrov went on a hunger strike for 51 days, losing 52 pounds in a futile effort to win an exit visa.