Backside gap exposure is a new method for fabricating three-dimensional structure using optical diffraction effect. In this paper, the inclined angle and dimension of SU-8 three-dimensional structure were simulated using Matlab and the corresponding experimental study was conducted. The results demonstrate that backside gap exposure is a feasible way to fabricate three-dimensional structure. Simulation and experiment results reveal the same regularity that the inclined angle of side wall depends on the size of the gap, and the dimension of microstructure changes with exposure time if the size of the gap is constant.