The imaging model of an optical camera without aberration can be equivalently expressed by its conjugate nodal plane model. A pinhole camera model neglects aberration of corresponding equivalent model, and also fails to represent translation error of flat glass, tilt error of optical axis as well as aberration of an actual optical system. All of these four factors give rise to image distortion. Based on this, a geometry model of image distortion is proposed by utilizing the property of conjugate nodal plane. By comparing with commonly used aberration model, this model can theoretically interpret their physical significances. The simulation of an actual optical system indicates that the relative radial distortion and relevant angle error both calculated by this model appears in the range of rational design parameters. Meanwhile, the phenomenon of asymmetric image distortion is seen. In this model, four unknown parameters needed to be recognized are relevant parameters of principal plane, tilt angle of optical axis and axial spherical aberration of flat glass. Theoretically, this geometric model can be used to correct image distortion with less parameters to be identified.