The angular spectrum theory is introduced to analyze optical diffraction lithography in thick film photoresist, and the reflection and transmission on the interface and the variation of dielectric coefficient in depth direction in thick film inhomogeneous resist are considered. A novel lithography model is put forward to simplify the analysis of optical diffraction, and the fast Fourier transform (FFT) algorithm is converted into convolution calculation to save computation cost. Numerical calculation results show that the reflection and transmission on the interface and the variation of dielectric coefficient in depth direction have important effects on the diffractive field intensity of thick film resist.