According to the lithographic characteristics of thick film photo-resist, a novel physical model is established. Then Fourier model method, mostly applied to study the diffractive properties of gratings is adopted to analyze the diffractive image field in thick film photo-resist. The factors such as reflection and transmission at the interface, complex diffraction in the photo-resist, are taken into account. This method has higher computing precision and smaller computation cost. Numerical simulation results show that the distribution of diffractive image field is relevant to that of complex refractive index. As a result, it is reasonable to regard the thick film photo-resist as grating with variable refractive index during exposure process.