SU-8 epoxy resin is a kind of polymer material of nonlinear optical properties. It is ultraviolet sensitive and its refractive index increases because of the photopolymerization. Finite-difference time-domain method is employed to simulate the process of photopolymerization, and the inner distribution of refractive index of SU-8 film is obtained. Both the process in the plane thin film and the thin film with a microlens are simulated. It is concluded that self-focusing effect which results from the refractive index change leads to the formation of self-written waveguide. The self-written waveguide consists of two continuous parts, one is cone and the other is cylinder. The length of the self-written waveguide increases with the time of exposure. Given the same exposure time, cone part in the SU-8 film with a microlens is more convergent than that without a microlens, and the length of the self-written waveguide in the SU-8 film with a microlens is longer than that without a microlens. The length of the self-written waveguide decreases with the increase of the ratio of the height to the basal diameter, given the same exposure time and a constant basal diameter of the microlens. The length of the self-written waveguide with the ratio 0.08 is 12.2% longer than that with the ratio 0.24, and the cone part with the ratio 0.08 is 19.2% longer than that with the ratio 0.24.