Parallel mechanisms with the capacity of large spatial localization are in important demand and difficult to design. In this paper, a 3-SPR parallel parameter is proposed to realize a large positioning workspace by using parasitic motion. In order to maximize the potential of the manipulator, optimum design is conducted. Firstly, kinematic analysis is carried out to investigate the characteristics of parasitic motion. Then, the motion/force transmission and constraint performance of the manipulator are evaluated, where an approach for computing the transmission and constraint indices is presented. The design indices are constructed to evaluate the size of the positioning workspace in the xy plane and the average level of the motion/force transmission and constraint performance. Afterwards, the parameter design space is established by the parameter-finiteness normalization method. On these bases, the region satisfying the design requirement is derived by the performance chart-based design methodology. The proportional line method is presented to determine the optimum workspace along the \(Z\) -axis. Finally, a group of optimum parameters is obtained, and the large spatial localization capacity of the manipulator is achieved. This work lays the foundation for the development of the 3-SPR parallel manipulator, and the methods proposed in this paper can be further applied to the design of other parallel manipulators.