Abstract: The air pollution by particulate matters emission from gasoline direct injection（GDI） engine attracted more and more attention. In order to reveal the soot formation mechanism in the cylinder of a GDI engine, an improved two-step process soot model was proposed and was applied to the Kiva-Chemkin software. A three-dimensional computational fluid dynamics（CFD） simulation was carried out to describe the process of spray impingement, mixture formation, combustion, and soot formation inside the cylinder of a typical GDI engine. The results show that, the soot of the GDI engine primarily came from stratified mixture and wall-film combustion, namely pool fire. In homogeneous mode, increasing the fuel injection timing allows better fuel atomization and more uniform mixture, thus reducing soot formation. However, if the injection advance angle is too large, it will lead to significant spray impingement, causing the amount of soot in cylinder increased dramatically. In stratified mode, soot comes mainly from the locally rich mixture. Too early injection timing is not conducive to produce spray-induced tumble and uniform mixture, and will produce local fuel rich area, resulting in a higher amount of in-cylinder soot. The results show that the proposed model can be used to predict the soot formation process in cylinder of a GDI engine.