Abstract: The authors investigate the differential kinematics and dynamics of the 3- RSR parallel mechanism, establish the kinematic constraint equations of the movable platform according to the structural character of the sub-chains, and on this basis obtain the closed-form velocity and acceleration formulas of the mechanism. Through disassembling the mechanism at the spherical joints and using the force or moment equilibrium relations of both upper links of the sub-chains and the combinative chain of movable platform and sub-chain upper links, the authors determine the constraint forces acting on the spherical joints. Finally, the analytic expressions of actuator driving moment are derived by means of the moment equilibrium of the lower links of sub-chains.