Dill’s classical exposure model on depth axis and time axis are generalized, respectively. On the depth axis, based on the Kirchhoff diffraction equation, complex-number-refraction-index was introduced, and the diffractive illumination distribution inside the resist at one moment was calculated by using the Beam Propagation Method(BPM). On the time axis, the characteristic of SU8 and its reaction course during exposure were analyzed, an appropriate chemical reaction kinetics model of the photocrosslinking reaction was built, and the diffractive illumination distribution at different moment was calculated. Finally, by building a kind of integrated exposure model, the diffractive illumination distribution after some exposure time was calculated. The results indicate that the illumination distribution variation with time inside the resist is not obvious and the impact of exposure time on illumination distribution is relatively low, which will be expanded during the post-exposure-bake process.