As high-speed railways are continuously being developed, the elastic resonance panels are gaining more attention as a way to reduce railway noise. To develop an elastic resonance panel for high-speed railway applications, the flow environment must be considered. In this study, the sound absorption performance prediction technique for elastic resonance panels considering the flow environment was established by applying an acoustic impedance model. By comparing the experimental and derived results from the established method, the validity of the technique was verified, and the sound absorption performance with respect to the design variables was analyzed based on the proposed technique. The analytical results of design variables confirmed both the degradation of sound absorption performance because of the flow environment and the movement of the absorption peak frequency. In addition, a design method based on the porosity was proposed, and the specific porosity is presented as the design reference porosity. Further, we observed that the increase in flow velocity inverted the movement tendency of the sound absorption peak frequency based on a specific porosity.