Robot working space is an important kinematic indicator. After the modular service robot arm was introduced, DH coordinate system was used to build kinematics equations of 5-DOFs services robot arm. The whole working space model of the robot system was then established. Monte Carlo method was used to simulate the working space, which can straightforwardly and visually describe the working space of robot with high speed. It was indicated that the working points in the workspace of the robot were distributed compactly and uniformly, which can meet design requirements with high efficiency. The effectiveness of the working space model was testified by the simulation analysis on Matlab and experimental validation.