The limbs kinematic screw and constraint screw of 3RPRR spatial parallel mechanism were analyzed based on the constraint screw theory. Three pure translational degrees of freedom (3DOF) of the mechanism were determined, and the rationality of its original input selection was judged. Position equation of the parallel mechanism was established by using DH method to build every branch connecting rod of coordinate system. In the last, the forward numerical solution for position equation of the parallel mechanism is obtained by using homotopy algorithm. 