In order to design a set of injection molds for the housing of electrical instruments that can realize automatic demoulding, Moldflow analysis technology is used to determine the optimal gate position. With the goal of reducing the warpage deformation of plastic parts, the best cooling scheme was determined through a comparative analysis. In order to realize the automatic separation of the gating system, the three-plate mold base is selected for mold opening. Combined with the optimization results, the gating system and cooling system of the mold were designed using UG NX software. According to the structural characteristics of the outer side hole of the plastic part, the slider is placed on the moving die side and the fixed die side in two forms, and the outer core pulling action is completed with the bent pin respectively. There is an inverted buckle structure on the back of the plastic part, and the inclined top structure is used for inner core pulling. The actual production verification shows that the mold structure design is reasonable, the purpose of automatic demoulding is realized, and the plastic parts have better molding quality and can meet the production requirements.