Abstract: The method and law of controlling the molecular weight of acrylamide-acrylic acid co-polymer prepared by γ-radiation initiation copolymerization were studied. The molecular weight of copolymer can be effectively controlled by the addition of different amounts of chain-transfer agent, by changing the absorbing dose rate, the pH in reaction system and the water-bath temperature, and by using various monomer concentration. Conversion of 97-100% can be reached; and water-soluble copolymer with an intrinsic viscosity (η) of 2 to 32 dl/g can be obtained under the condition of mole ratio of the carboxyl group to the amide group being 29/71.