Abstract: To simulate more accurately the effect of the transmittance of the multi-molecular contaminated films on the susceptible optical components, a model is established based on the Lorentz-Lorenz relation and the theory of the wave propagation in the hierarchical dielectric film, with consideration of the transition layer. For a comparison between the numerical analysis and the experimental testing, the non-metallic bilayer films are prepared on two kinds of glass substrates to carry out an outgassing test based on the ASTM E 595 standard, with the measurements of the transmittance and the molecular species by the spectrophotometer and the mass spectrometer, respectively. It is shown that the calculation results obtained from the model agree well with the measurement values, thus the model is validated.