The error analysis was studied for the parallel mechanism in order to improve the positioning precision of the 3SPR mechanisms end effector. The error model was established for by complete differentialcoefficient matrix theory, the relations of input and output error were deduced. The sensitivity of error model was established and the graphs of which the input error sources to the influence degree of the localization accuracy were drawn. The results show that the actuator length errors and the errors of the moving platform along Z axis should be controlled strictly in the course of the design, manufacture and assembly of the manipulator, which can improve the positioning precision of the institutions, according to the 3σ principle, the expected error in a workspace volume which is caused by the error source in the scope of the standard deviation of the tolerance can be calculated, those conclusions have a certain guiding significance for the manufacture and installation of the parallel mechanism.