Mechanical infrastructure should have good economic efficiency under the condition of sufficient rigidity, strength, and stability. Therefore, box structure with inner stiffener plates is usually applied for mechanical infrastructure. Empirical design method and multi-objective topology optimal design were carried out respectively on layout of inner stiffener plates of a sample mechanical infrastructure, and two design schemes were obtained. The static and dynamic mechanical properties by the two design schemes were compared. The result showed that multi-objective topology optimal design is much better, by which the rigidity of the optimized structure is improved, the weight of the optimized structure is reduced by 11. 21%, and the first natural frequency is increased by 25. 07% by compared with that of the empirical design method.