Aluminium alloy die casting has been widely used in the development of new energy vehicle parts due to its advantages of lightweight and high-efficiency production. However, considering the high performance requirements of castings in the automotive field, it is necessary to design a reasonable die-casting scheme to effectively control common die-casting defects such as gas porosity, inclusions, and shrinkage holes to meet product quality requirements. In this paper, taking the die-casting parts of automobile rear suspension beams with uneven wall thickness as the research object, the two gating system schemes were compared by numerical simulation, the casting defects existing in production were analysed, and an improvement scheme was proposed. The results show that the five-gate scheme can shorten the filling distance of molten aluminium and reduce defects such as entrained air and oxidation inclusions. Aiming at the shrinkage hole problem of the two thick walls far away from the inner gate, a scheme of inserting copper alloy blocks in die was proposed. The numerical simulation results show that the cooling rate of aluminium melt at the thick walls can be accelerated by a high thermal conductivity CuCoBe mould insert, and the hot spot and shrinkage holes can be effectively eliminated.