In order to reduce the mass of a rocket launcher effectively and improve the traversing mechanism's maneuver performance, an optimization design for the structure of the rocket launcher's rotary cabinet based on density variable method of topology optimization was studied. In four typical load cases, the structure stiffness requirement was taken as the constraint and the minimum mass as the objection, the best material distribution pattern of the cabinet structure was acquired. Then the analysis of the optimized structure was carried out. The result shows that the mass of the rotary cabinet is reduced by 15.4% after optimization; the stiffness and the strength of the structure meet the design requirements. The method of topological optimization has important reference value for improving other structures of the rocket launcher.