Complex structures such as the oil pipeline and reinforcing ribs from the case of 9AT automobile transmission are concentrated on the right side, resulting in poor symmetry and high requirements for airtightness, which lead to great molding difficulties and low production qualification rates. The injection velocity is a key factor affecting the product qualification rate. In this paper, the process parameters were determined through calculation, and the effects of three different flow rates on the metal liquid flow and defect rate were simulated. The results show that when the pouring temperature is 680 ℃, the injection speed is 3.5 m/s, and the mold temperature is 200 ℃, the flow rate is the gentlest, creating the smallest volume of shrinkage porosity and shrinkage cavity. Die casting experiments were carried out with this set of parameters, the microstructure and morphology of the castings were observed, and performance tests were conducted. The tensile strength and elongation are 257.3 MPa and 4%,respectively. The performance and quality of the transmission case product meet the use requirements, and the qualified rate of the products increases from 65.3% to 77.5%.