Mainly aiming at optimizing and improving the scalar wave approximation method, the scalar wave approximation method is used to simulate the transmission spectrum of the three-dimensional photonic crystal. The basic idea and calculation steps of the scalar wave approximation method are introduced. Then we optimize and improve this method. The research on the photonic band gap is emphasized. The experimental data are compared with the simulation result in order to verify the accuracy of the method. The multi-layer three-dimensional photonic crystals are prepared by the fluid chemical vapor deposition method and their transmission spectra are measured. The experimental results show that the scalar wave approximation method can simulate the optical behavior of the three-dimensional photonic crystal.