Based on the finite - difference time - domain ( FDTD) method and combined with Matlab programming，the characteristics of light reflection，transmission and interference at the interface of discontinuous medium are studied. Parameters such as the gradient and continuity of permittivity，which affect the intensity of reflected light at the interface，are discussed. Also，dynamic modeling of light interference is rebuilt on computer and systematically investigated by tuning the electromagnetic and structural parameters of the interference thin film. The FDTD method can achieve a time-domain and numerical solution of the electromagnetic field，which is very convenient for dynamic demonstration. Meanwhile，computer experiments can be realized by changing the structural and electromagnetic parameters of the cared medium，which makes it possible for students to do it themselves and to learn，to investigate the physical nature that is covered behind of those numerical physical phenomena.