Abstract: Based on the screw theory,the constraint screws acting on the moving platform can be got when the pairs,which can be acted as actuating input pairs in all limbs of the parallel machine tool(PMT) ,are locked. The same amount of constraint screws as the degree-of-freedoms of the mechanism are selected to constitute the constraint matrix with the structure constraint screw inherent in the mechanism. The rationality of input selection can be judged by the rank of the constraint matrix. The constraint force/torque dexterousness is defined as the ratio of minimum singularity value to maximum one of the force/torque constraint matrix,by which the performance of the combinations of actuating inputs are evaluated. The result of actuating input selection shows that there are 5 kinds of reasonable actuating input combinations in 5-UPS/PRPU PMT,and the actuating input combination of the 5 prismatic pairs in UPS limbs is the best one.