A bridge with a main girder of π‐shaped sectional configuration is concerned in this paper. Nonlinear properties of mechanical damping with respect to motion amplitude are first determined by wind tunnel tests of an elastically suspended section model.On this basis，flutter thresholds and nonlinear post-flutter vibration characteristics with torsional-bending frequency ratios of 0.872，0.971，0.988，1.035，1.085 and 1.245 are investigated. The results show that the motion always ends up with limit cycle oscillations（LCO）beyond the flutter thresholds，and the LCO amplitudes of post-flutter increase with the wind speed. The torsionalbending frequency ratio has a substantial impact on the aeroelastic response of the model in many aspects，including flutter threshold，post-flutter LCO，evolution path of the post-flutter LCO with respect to the wind speed，extent of coupling between the vertical and torsional motions，and even the“soft”or“hard”nature of the flutter，etc. The vertical and torsional motions are coupled at the same frequency but with a non-zero phase angle which varies significantly with wind speed and torsional-bending frequency ratio in the post-flutter state. Due to the influence of aerodynamic stiffness，the frequency of the model torsion decreases monotonously as the wind speed increases. However，coupling occurs even when the initial torsional-bending frequency ratio is less than 1.0，which differs obviously from the classical coupled flutter characteristic of streamlined sections. This paper also shows that the current design code for wind resistance of highway bridges is insufficient in dealing with the influence of torsional-bending frequency ratios.