Abstract: With the 1D simulation model of a multi-hole injector which was constructed by Hydsim code, the injection parameters during the needle opening and closing at different pressure were studied,then the spray formation at 15 MPa injection pressure was simulated by FIRE and its numerical model was validated in a constant volume bomb. Based on this, the influence of tumble ratio and injection timing on low-speed mixture formation process for a turbocharged gasoline direct injection engine was analyzed. The result showed, for full load and 2000 r/min, the higher tumble ratio can strengthen cylinder air flow, subsequently the faster evaporation of fuel and the more uniform mixture can be achieved, meanwhile the appropriate start of injection can decrease fuel impingement and also improve the distribution of mixture concentration, all of these are better for improvement of combustion and engine performance.