In order to meet the surface shape detection requirements during the grinding stage of large aspheric mirror, the sources of angle error are analyzed, and one method for correcting angle measurement errors based on the S polynomial fitting is proposed. This method can effectively overcome the impact caused by the sampling location and random errors. As for the angle errors at different frequency bands, the generatrix and composite sampling methods are proposed, and the generatrix sampling method can suppress the random error within 10% in the calibration experiment. The above correction method is proved to be possible, which provides a theoretical basis for its application in the future experiments.