-In order to apply the robot to medical institution, a mobile medical service robot arm is designed. Firstly, according to the design requirements and the ergonomic theory, the various configurations of the mechanical arm are analysed. The optimal configuration of the mechanical arm is obtained by using the global relative manipulability value as the index. Secondly, according to the application and function of mobile service robot, the maximum working space distance index, static stiffness performance index and global velocity fluctuation performance index of arm performance are proposed. On this basis, the mathematical function model of robot arm size optimization is established. Finally, according to the characteristics of the multi-objective and multi-constraint of the optimization function model, the multi-objective particle swarm optimization algorithm based on the Pareto dominance principle is chosen as the solving method. The results show that the size optimization is very effective in reducing the speed fluctuation of the mechanical arm during motion. Therefore, through the combination of topology optimization and size optimization, the mechanical arm has the characteristics of high stiffness, light weight, flexible and stable.