Theoretical analysis and experiment study on the pose error compensations of parallel mechanisms have been presented. A method for error compensations by means of differential operators and electro-optical collimation system is discussed. Compensation quantities oriented to main error sources from position coordinates of joints on the movable platform can be obtained through mathematic models. Compensation quantities provided by driving legs can be realized precisely by linear calibration techniques. Orientation angles can be measured by means of electro-optical collimation system and a three-dimension turning table, which is centrally positioned and can complete multi-direction inverse adjusting. Numerical example relevant to Stewart platform has been given. Simulation and experiment results indicate the feasibility of the method.