The constraint reaction is existed in a patallel mechanism with fewer degrees of freedom (DOF), which leads to its deformation. The whole Jacobian matrix containing drive and constraint was set up by 3SPS+SP parallel mechanism. By considering the structure model of constraint reaction influence on the mechanism, the mapping relationship between input driving force and binding reaction and external load was derived. The whole of stiffness model was established，and the relationship between moving platform linear translation and angle rotation deformation with the external load was obtained. The stiffness evaluation indexes were defined. The changing rule between main structural parameters and the global stiffness evaluation index are analyzed, which laid the groundwork for the structural parameters of mechanic optimization.