The analysis of the planetary roller screw (PRS) mechanism is carried out based the theory of spatial screw. The kinematic sketch of PRS mechanism is established with consideration given to the characteristics of the spatial screw mechanism. The degree of freedom (DOF) of the PRS is calculated according to the modified G-K formula, and the effects of all the elements of the mechanism on its DOF are investigated theoretically. The DOF of the mechanism with different numbers of rollers and rings is calculated and analyzed. The comparative analysis of the DOF of the mechanism with single roller and ring shows that the increase of the number of rollers and rings has an effect on the kinematic characteristics of this mechanism. The theoretical analysis reveals the principles of the PRS kinematics to a certain extent.