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During the cantilever construction stage, long-span sea-crossing twin-box cable-stayed bridges exhibit low stiffness, low natural frequency, and low damping ratio compared to the completed bridge state, making them susceptible to large buffeting responses under the high design wind speeds typical of coastal areas. Accurate calculation of the buffeting response during the bridge construction period is essential for enhancing construction safety. In this study, China’s Huangmaohai Bridge is used as a case study to introduce the three-dimensional aerodynamic admittance function (3D-AAF) of a twin-box into the buffeting analysis theory for long-span sea-crossing twin-box cable-stayed bridges. The 3D-AAF and the spanwise coherence function, key parameters of the buffeting force, were obtained through large-scale section model tests. These parameters describe the unsteady and spanwise distribution characteristics of the buffeting force. The self-excited force was calculated using the Scanlan model. Subsequently, the buffeting response of the bridge was calculated and compared with the results from the full bridge aeroelastic model test. The results demonstrate that the theoretical calculations are in reasonable agreement with experimental values, proving that the frequency domain buffeting analysis method based on 3D-AAF for long-span twin-box cable-stayed bridges is reliably accurate. [ABSTRACT FROM AUTHOR]