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The flutter responses of the Nanjing No.4 bridge flat box girder under different wind attack angles were tested in detail through sectional model tests. The evolution of unsteady and steady critical amplitudes at different wind speeds was discussed. Based on the amplitude envelope of the flutter response and the Hilbert transform, the amplitude-dependent modal damping of the system was identified, and the mechanism of the flutter mode change with the wind angle of attack was initially explained. Secondly, the modal parameters of the system under different wind attack angles were extracted. With the bimodal coupled flutter analysis method, the nonlinear flutter derivatives of the section under different wind attack angles were identified, and the change law for the amplitude dependence of the key flutter derivatives on the wind attack angle and the potential influence on the section flutter morphology and characteristics, were studied. Finally, the effects of the wind attack angle on the uncoupled and coupled aerodynamic damping were analyzed through analyses of the modal damping subterms one by one, and the dynamic mechanism of the differential flutter performance caused by fractional damping was illustrated. [ABSTRACT FROM AUTHOR]