The deformation of optical mirror surface includes rigid-body motion and surface error (distortion). Analyzing the distortion and rigid- body errors plays an important role while evaluating the imaging quality, the environmental adaptability and spatial positional stability of the opto-mechanical system. Removal of rigid-body errors is based on the coordinate transformation method. Three methods are described to perform the calculation of root mean square (RMS) value and peak valley (PV) value of distortion. The optical model in ZEMAX software is established through grid sag surface by cubic interpolation algorithm. The RMS spot radius calculated in ZEMAX software is used as the optical standard of mirror distortion. The linearity between RMS value and PV value of distortion and RMS spot radius is taken as the evaluation criteria of the optical performance of three statistical methods for mirror distortion. The results show that the information of distortion calculated by surface fitting method is not complete, the surface fitting method can be applied only when parameters of the surface equation have not been significantly changed. However, the information calculated by normal direction and optical axis direction methods is complete which can be used to measure the image quality of optical system comprehensively.