Use Pro/Engineer software to draw three-dimensional solid models of the lower case of the gearbox with two different gating systems. Based on the better pouring scheme, study the effect of pouring temperature, mold temperature and filling pressure on the low pressure casting process of the lower case of AlSi9Mg gearbox. The influence of liquid filling and solidification process. ProCAST software is used for numerical simulation, and the test results are effectively analyzed with a standardized orthogonal table. The better process parameters are as follows: pouring temperature 700 ℃, mould temperature 350 ℃, filling pressure 45 KPa. Under this process parameter, the defect volume of the lower shell casting of the aluminum alloy gearbox is reduced from 1.537cc to 1.425cc.