Taking classic 3-RPS and its variation institutions 3-SPR parallel mechanism as an example,this paper establish two institutions forms of jacobian matrix with 6×6 form considering constraining anti-forces,and establish the stiffness model.The entity model of two parallel mechanism were established by using SolidWorks software,and the finite element analysis were carried out by using Workbench software,this paper analyzes the deformation situation of two parallel mechanism.The results show that the stiffness of 3-SPR was better than 3-RPS parallel mechanism and the stiffness of Z-direction has the better performance than the other two directions.