Arm is the main bearing part of a certain armed reconnaissance robot weapon station and makes up most of its weight. In order to achieve the reliability and weight reduction, the finite element optimal design is carried out by OptiStruct and HyperMesh. First of all, the original structure is analyzed under three typical shooting angles by the means of finite element to discover the effects of load on static structural strength and displacement. On the basis of that, a new topology optimization model, which is based on a smaller-density material-the 6061 aluminum alloy, is established to replace 45 steel. Regarding the relative density as the design variable, with the objective function of minimizing the volume, the optimization model is attained. By comparison with experimental prototype, the optimized scheme has completed a more reasonable structure under the prerequisite of satisfying the strength requirement, greatly reducing the weight of the platform and making it easier and lighter to carry and deliver in the war. All in all, this research can provide a theoretical and engineering reference to the optimal design of the same type of small operational platform.