The mounting eccentric error is one of the largest error sources for circular grating angle sensors (CGASs). To improve the measuring accuracy of CGASs, one error compensation method based on mounting eccentricity is proposed in this paper. The mounting eccentricity and measuring error of CGASs was modeled after analysis. Six joints mounted with CGASs were used in the eccentric error compensation experiment. The rotation error of the joints was calibrated with a coordinate measuring machine and a gauge block. Then with the calibration data, the parameters of the eccentric error compensation model were calculated. The experimental result shows that after error compensation the rotation angle errors of the joints are reduced about 58 to 543 times.