The coupling relationship between the position coordinate and the attitude angle of the 3-RPS parallel mechanism is obtained from the structure constraints of the platform. Thus an analytic expression of the workspace boundary is derived. The relative motion of a member connected by a motor can be described by motion screw. According to the property of the reverse helix,the reverse helix of the branch motion screw system is obtained,which represents the structure binding force of the branch on the moving platform. In turn,the singularity of the mechanism is transformed into the analysis of the linear correlation condition of moving platform motion screw system and the constraint screw system. Therefore the singularity of mechanism is divided into kinematics singularity and constraint singularity. The physical essence of the influence of the two kinds of singularity on the performance of the mechanism is given,and the analytical expression of the singular configuration is derived. An example is given to show the distribution characteristics of the working space and the division of the working space.