For the absorbing-scattering semitransparent coatings with smooth surface and diffuse opaque substrate, the propagation model of the incident spectrum was derived and the spectral radiative transfer was solved by Monte Carlo method. The effects of the optical thickness, medium scattering and substrate reflection on the bi-directional reflection of the coating surface were analyzed. The results show that the reflection of an absorbing-scattering semitransparent coating is obviously anisotropic even if the Fresnel reflection on surface is not taken into account, and its BRDF is quite different from that of an absorbing coating.