Abstract: By comparison of the characters and compositions of Tahe heavy oil before and after aquathermolysis with or without viscosity reducer and a model compound aquathermolysis reaction, a mechanism of viscosity reduction was established. The test results show that the viscosity of Tahe heavy oil can be reduced up to 65.8% by catalytic aquathermolysis, 32% higher than that of aquathermolysis without viscosity reducer. The heavy component is decreased, the light one is increased, and the molecular weight and the sulfur content are reduced after treatment. The aquathermolysis experiments of dodecyl mercaptan and dioctyl sulfide with viscosity reducer indicate that the bond breakage of C-S, S-H，C-C，S-S are happened. The reasons for the irreversible viscosity reduction are the weakened molecular interactions like hydrogen bond and coordination bond and bond breakages after treatment, resulting in smaller aggregation of resins and asphaltenes and thus the reduced viscosity.