Abstract: The method of correcting the retardation deviation of a wave-plate through geometrical optics is studied.By means of exploring the relation of the orientation of incident light and the change of retardation of the crystal device,the formula for calculating the change of retardation of a wave-plate is derived when the incident light is oblique incident.Thus the retardation deviation can be corrected through rotating the wave-plate on the axis direction parallel to the optical axis or the axis direction vertical to the optical axis.The retardation of a λ/2 wave-plate and a λ/4 wave-plate is corrected by means of light extinction method and light minimum method respectively.Thus the deviation of retardation of wave-plate can be corrected by means of rotating.