Based on the scattering and refraction characteristics of optical waves in medium, the superposition mechanism of pulse trains after interface reflection and medium scattering is researched. Analytical solutions of the transmittance and reflectance of pulse trains in non-scattering semitransparent slab are deduced. The discontinuous finite element method and time shift and superposition method are used to calculate the transmittance and reflectance in one-dimensional scattering slab medium. The results show that the pulse train, reflected by interface and scattered by medium is split into a series of sub-lightwaves with different time delays due to the interaction of incident light and medium. After passing different optical path, the sub-lightwaves are superposed as the reflected and transmitted light waves leaving the slab. The superposition effects vary with the variation of scattering coefficient, pulse width and time interval. Regulating pulse width and time interval of pulse trains will produce the reflected and transmitted light waves with different wave shapes.