Basing on the introduction of working principle and structure of sphere cam engine, the variation of cylinder volume was studied. Furthermore, the theoretical thermodynamic model of engine was established to analyze the combustion process of sphere cam engine. Then the theory result was compared with the simulation result in FLUENT. The oil spray process was studied in detail. Results show that the calculated pressure curve and temperature curve in cylinder are similar to the simulation results, which validates the validity of theory model. However, the vortex of sprayed oil observed in FLUENT can lead to the rich oil phenomena, which may decrease the combustion efficiency. 