Topology design optimization method of rib layout in box structure is studied by utilizing the structural topology design optimization approach. The box structure is simplified to a hollow box with some parallel bars, two ends of which are connected with the outer loaded panel and the supported panel. The outer loaded panel is set to be the optimal design area, and the density method is applied to solve the topology optimization problem. The optimal material layout of the outer loaded panel decides the existence of the inside bars, and the existed bars form the rib plates. The validity of the suggested method is verified by some typical design problems. The results show that the obtained box structures have much better mechanical and economical performances by comparing with traditional structures, and because the 3-dimentional design problem is changed to a 2-dimentional problem reasonably, the suggested method is effective and has low computational cost.