To set up the evaluation base and method of medical rigid endoscope distortion, by using optical axis symmetrical spherical Z view field as the model to simulate concave characteristics of body cavity, the analysis base of endoscope object-image conjugation is established. By using optical principle together with the definition principle in ISO 9039:1994 “Optics and optical instruments——Quality evaluation of optical systems——Determination of distortion”, with definition of distortion determined by clinical requirement of theoretical distortion, the distortion conversion formula of different sphere Z shapes is deduced by main ray trace principle. The medical rigid endoscope distortion definition and expression which are expressed as unit relative distortion, and distortion conversion formula and testing method of conversion of view field surface, are all established. The distortion evaluation base and method established are applicable for the medical rigid endoscope standard.