Based on the Fresnel diffraction theory, a theorictical model of the millimeter wave holographic imaging radar is developed, in which the relationship about the irradiator, transition space, object back scattering coefficient, back scattering transition space and the received complex data is created. Using the model, the object image can be obtained. A simulation program is presented by the Matlab program based on the Fourier optics angle spectrum theory, which can be used to analyze the effect of each factor on the imaging quality and help design realistic radar system.