With the purpose of optimizing the combustion system of a homogeneous charged gasoline direct injection engine, its working process was simulated with a 3-D CFD software package. The authors investigated the influence of injection timings and combustion chamber shapes on the charge motion, fuel injection and their influence on fuel-air mixture formation. With the comparisons of fuel-air mixture quality and combustion processes between two typical injection timings, the optimal injection strategy was chosen. To further improve the mixture quality, configuration on piston crown was optimized and analysis on mixture formation processes of the new configurations was made. Simulation results indicate that the new configurations have faster fuel evaporation rate and better mixture homogeneity.