In order to solve the problem of weak stiffness of the external cradle body of the heavy machine gun tripod, the external cradle body of the tripod is optimized. According to the improved simplified model, the free mode analysis and constrained modal analysis are carried out. From the light weight design point of view, the structure optimization and wall thickness improvement of the outer rocking frame are carried out. This paper presents a design scheme of external rocking frame which can not only reduce the mass of the external frame but also increase the integral natural frequency of the tripod. The results show that the natural frequency of tripod is improved obviously.