A new kind of constraint error branched chain of 3SPS/S parallel mechanism of robot was introduced. In order to improve the positioning accuracy of terminal platform, the error of this parallel mechanism was analyzed. Firstly based on the kinematics inverse solution, the differential equations of motion for driving chain were set up, and the mathematical model of positive solutions of output poses error of the institutions was established. And the error in a given institutional conditions, the output pose error changes in the process of terminal to perform the work were considered. By using the idea of orthogonal experimental design to equalize distribution of parametric error level, the accuracy of this mechanism was analyzed, the error distribution histogram in one pose and reliability of allowable accuracy range were drawn. The results show that, the error values can be quickly calculated by using orthogonal experimenatal method, which establishes theoretic basis for accuracy design of the parallel mechanism and calibration of kinematics parameters.