Aiming at the 3TIR spatial parallel mechanism, this paper proposes a widely-generalized discrete Monte Carlo method. Through the discretization of rotational degrees of freedom, the dimensionality reduction effect is achieved. By using the union and intersection of subspaces, The reachable workspace of the entire space parallel mechanism and the visualization effect of the flexible workspace are analyzed by taking an example of a 3T1R space parallel mechanism of type 4-PRPaRR as an example to study the effect of the specific parameters on the size of the workspace and the performance. For the first time, the work performance of space parallel mechanism is analyzed from the perspective of flexible work space and reachable work space, which provides an important theoretical foundation for the application and research of multi-freedom space parallel mechanism.