A Harvard Law School professor revealed in Moscow yesterday a Russian claim that the Soviet Union's prison population has decreased by 70 percent since the death of Joseph Stalin.

Professor Harold J. Berman, in Russia to do research, quoted the deputy prosecutor general of the Soviet Union, P.I. Kudryavtsev, as his source. The Russian said that only two percent of the prisoners released were political prisoners.

Berman said Kudryavtsev told him that more than half of the total Soviet prison population was released immediately after Stalin's death under the amnesty law of 1953. All sentences passed during the last 25 years have been or are being reexamined, even those already completed, Berman was told.

Repeated reports of release of prisoners and abolition of concentration camps have come up this year, but no estimates of totals such as Berman obtained. He quoted Kudryavtsev as saying that two-thirds of the concentration, or labor, camps have already been liquidated.

Most of the political prisoners released were accused of wartime treason and collaborating with the Germans, Berman was told. These steps are all part of the general Soviet program of liberalizing in some degree the harsh Stalinist penal system.

Berman is in Europe on a sabbatical leave to continue work he has been doing for several years on the structure of Soviet and East European trade. From his home base at Geneva, Switzerland, he has taken trips to Yugoslavia and France, before going to Russia several weeks ago. In Moscow, Berman has conferred with several Ministry of Justice officials.