The　dynamic　modeling　method　for　parallel　robot　based　on　the　principle　of　virtual　work　and　equivalent　tree　structure　is　proposed　by　taking　off　the　platform　and　the　chains　as　well　as　degenerating　parallel　robot　into　a　tree　structure,　the　closed-form　solutions　for　the　inverse　and　forward　dynamics　models　of　parallel　robot　are　derived.　The　method　is　applied　on　kinematics　and　dynamics　analysis　of　a　representative　3-RRR　spherical　parallel　robot.