Abstract: Nowadays, the problem of thermal load of engine had become a main obstructor as it developed, which made it become an urgent issue. The heat transfer simulation in this paper could be achieved by adopting liquid-solid coupled algorithm which was applied to engine block and cooling water jacket, consequently, the boundary conditions which were difficult to confirm when simulating separately were transformed into internal boundary conditions, then the heat transfer simulating was carried out by the Fluent software. The convergent results were consistent with the data measured by thermocouples in cylinder block and cooling water jacket. Based on the temperature distribution of the calculation, some improvements on the structure of engine block and water jacket were made to enhance the cooling performance. The numerical simulation of heat transfer was done separately again, we found that the highest temperature of the two improved engine block model A and B was reduced to 152℃ and 122℃ when compared with the primary model, then confirmed the rationality of new designed structure. Finally, it effectively improves the thermal load of the engine.