Abstract: The mathematic model for distortion is discussed. The distortion measurement with a precision goniometer and a bench comparator is introduced. The pixel distance equivalent of the CCD detector was calculated by the magnification method. The centroid of point target was derived by the centroid method. The pixel distance between the centroid and the origin on the image plane was multiplied by the pixel distance equivalent to achieve accurate distance. The absolute distortion was acquired by tangent function. The equation was worked out to calculate the coefficient. The results of the calculation and data processing indicate that the fitting effect of the distortion mathematic model is satisfactory in medium field of view.