For parallel robots with general configuration, it is difficult to achieve the large rotation range requirement for pelvic fracture reduction. Application-based workspace optimization is important. In this paper, a 3-revolute-revolute-prismatic-spherical (3-RRPS) parallel robot with optimized parameters is studied, which can offer a large orientation workspace and relatively compact configuration. The inverse kinematics of the robot is analyzed by the closed-loop vector method. The cylindrical coordinate searching algorithm and boundary extraction method are adopted to calculate the orientation workspace. Defining the comprehensive branch length (CBL) to express robot’s compactness under the premise of satisfying the required orientation workspace. The genetic algorithm (GA) is adopted to optimize structural parameters of the robot with CBL as the objective function. The optimization results show that when the orientation angles of the moving platform are limited to not less than 28 deg, the CBL of the robot is 291 mm. Finally, the virtual prototype simulation verification shows that the orientation angles of the moving platform around x-axis and y-axis can reach ±28 deg, the orientation angle around z-axis can reach ±40 deg, which meets the requirements of the rotational range for pelvic fracture reduction surgery.