In production,the Φ105 mm hole of rear oil seal bearing die castings often appear porosity, shrinkage and other defects , resulting in the parts to scrap. The existing gating system design of rear oil seal bearing die castings was improved , and the flow and temperature field of the two programs that before and after improvement were analyzed by ProCAST software. The simulation results predicted the casting defects, and the conclusion was consistent with the production. By using the improved process optimization, the rejection rate of die castings can be reduced effectively. Die casting is a chipless forming of casting with high casting dimensional accuracy, high surface quality and high production efficiency [1,2]. Therefore, it has developed rapidly in the automotive, communications and other industries. However, due to shrinkage, porosity and other reasons, leading to high rejection rate of castings. With the emergence of commercial casting simulation software, the casting production has been developed from semi empirical and semi theoretical onto theorization, quantification and controllable track[3-5]. In this paper, the casting defects of rear oil seal bearing is analyzed by using numerical simulation technology. By optimizing the casting process, the casting quality is improved, and the rejection rate is reduced effectively.