Abstract: Low toxicity reagents including n-hexane, dichloromethane, ethanol and mixed solvent were used in the separation of heavy oil. The heavy oil in which the asphaltene was removed was separated into saturated hydrocarbons, aromatics and nitrogen compounds before and after reactions. The optimized conditions of each step, and the data about each component before and after catalytic aquathermolysis of heavy oil by using GC-MS assisted with the ¹HNMR and elemental analysis were obtained, and the changes of carbazole compounds in the resin were investigated. The results show that the nitrogen-containing groups are turned into some carbazole fragments. Meanwhile, the composition and structure of resin and asphaltene are changed. The supramolecular structure is damaged to some extent, generating two or three benzene rings and a number of small straight-chain hydrocarbons. These changes cause the viscosity reduction of heavy oil.