Based on the phenomenon of the s-polarization extraordinary optical transmission through subwavelength metallic grating on a dielectric film, the same phenomenon in bilayer metallic nano-grating has been found. In order to analyze the s-polarization transmission in this specific structure, the rigorous coupled-wave analysis and finite-different time-domain method is applied: the former is used for analyzing the transmission of the structure exactly and the latter is used for acquiring the optical field distribution of the structure. Using the equivalent refractive method, the equivalent mechanical model of the bilayer metallic grating is founded, which is as much of extraordinary optical transmission as the original model, to discover the relationship between the polymer and the s-polarization transmission. The comparison of distribution of field-intensity for two bilayer structures, with or without the polymer, illustrates that the existence of the polymer is the main reason to the s-polarization transmission peak appearance. Because the existence of the polymer can be treated as a waveguide and the s-polarization is coupled by metal grating and then turns to a surface wave, there is a resonant phenomenon occurred in the polymer area under the incident light with particular wavelength. In addition, the effect of geometrical parameters of the polymer, such as the refractive index and the thickness of the polymer, the effect of the thickness of the metal film on s-polarization transmittance are discussed. Increasing the refractive index of the polymer leads to the red shift of transmission peak both in the original bilayer model and the equivalent model, which indicates that the two models have the same property. The transmission peak can be explained by the Fabry-Perot-like resonance, and the red shift of transmission peak is result from the change of the resonance condition due to the refractive index increase. The polymer thickness increase results in the addition of the resonance modes and the corresponding transmission peaks. The cycle of the peak is calculated and the result is similar to the length of the Fabry-Perot-like cavity. However, the thickness of metal layer does not impact the position of the s-polarization transmission peak. In conclusion, the polymer which sustains a waveguide elecromagnetic mode is necessary for the extraordinary optical transmission, and the existence of Fabry-Perot-like resonance in the polymer film is the main reason of the resonant peak appearing.