Aquathermolysis is one of the thermochemical reactions in the heavy oil thermal recovery process. The key issue, i.e., the high temperature required by aquathermolysis, can be reduced by the involvement of catalysts. How-ever, the effective working temperature of catalysts is still above 240 degrees C, which needs to be further reduced to enhance energy efficiency. Considering that biochar has a high specific surface area, abundant functional groups, a wide temperature window, and can be easily modified, a series of ZrO2-MoO3-based catalysts loaded on raw or three modified lotus stem biochar were studied to investigate heavy oil catalytic viscosity reduction at low temperatures. Physicochemical properties of catalysts were investigated by a variety of characterization methods. The performance of four kinds of catalysts and the influence of reaction conditions (temperature and time) on aquathermolysis were evaluated. The results showed that silane coupling agent KH570 modified biochar (KH570-MLSB) has a higher specific surface area and more functional groups, which help the loading of active components and the dispersion in heavy oil. ZrO2 can form coordination compounds with S atoms in C-S, which greatly weakens the bond energy of C-S and reduces the activation energy of the aquathermolysis. The coordi-nation bonding energy with S of Mo is weaker than that of Zr. The main role of MoO3 is to inhibit the formation of coke. Therefore, catalyst IV, which consists of KH570-MLSB, ZrO2, and MoO3, has the best catalytic effect at 200 degrees C. In addition, the optimum reaction time of the catalyst is 24 h.