The rigid-flex coupled multi-body dynamic model of Motoman HP20 was built in this paper. The parts geometry shapes were modeled in 3D modeling software and imported into the multi-body platform. Then the joints were added to the parts. The arms were analyzed in FE software and modal neutral files were obtained. Then rigid parts were replaced by the modal neutral files. Driven curves of each arm joint were obtained by D-H method. The modal analysis of system was also made to analyze the robot dynamic characters. The results give some suggestions for robot motor selection and structural dynamic optimization.