This study conducted a CFD-2D coupled analysis of a suspension bridge subjected to wind loads. Previous studies found that rotational oscillation was due to differences in the restoring force at hanger cables and could generate torsional oscillations. However, due to uncertain external force terms, the previous studies could not be applied to analyze actual structures. To enable application in a real design process, this study proposed a methodology for determining the external force terms. The external force terms were determined with CFD, and a moment force term was added to equations of motion derived from dynamic equilibrium conditions. All constants and properties were calculated from an assumed cross section of superstructure. This methodology can be used not only to avoid torsional resonance but also in preliminary analysis in the bridge design stage.