Abstract: In order to solve the problem of symmetrical scoliosis and reinforcing rib on the inner top of the circular box cover, a dog-leg cam mechanism was designed to solve the problem of interference between the side core-type and the product. Due to the dog-leg cam mechanism on both sides of the product, the layout of the 4-cavity of a mold is arranged as a font to reduce the perimeter size of the mold, but it brings the problem of unbalanced filling system. By changing the runner layout, the pouring system is balanced. Through CAE comparative analysis, the filling time difference, pressure difference, temperature difference and other indexes of the balanced flow channel are significantly reduced, and the filling consistency is greatly improved. Finally, four parting surfaces are designed, including the pull point gate, eject the runner, the main parting and the product release, which optimizes the overall structure of the mold, saves the space for the parting and reduces the thickness of the mold.