Abstract: To analyze the three-degree-of-freedom (3-DOF) coupled nonlinear flutter characteristics of the main girder section and to investigate the influences of lateral DOF on the nonlinear flutter characteristics, a typical box girder section was taken as an example, and a 3-DOF coupled nonlinear flutter analysis method considering 18 amplitude-dependent flutter derivatives is established. The numerical simulation technique of forced vibration is used to identify 18 amplitude-dependent flutter derivatives of the main girder section under different reduced wind speeds and different amplitudes. Based on the identified amplitude-dependent flutter derivatives, the nonlinear flutter responses of the box girder section are calculated by the 3-DOF coupled nonlinear flutter analysis method established, and which are also compared with the numerical simulation results of free vibration, verifying the accuracy of the identification results of amplitude-dependent flutter derivatives and the nonlinear flutter analysis method. The influences of lateral DOF on the nonlinear flutter is quantified and its mechanism is revealed. The analysis results show that the participation of lateral DOF reduces the flutter critical wind speed and increases the steady-state amplitude of the nonlinear flutter, and that the larger the amplitude, the more obvious the increase is. The main reason of the results is that the introduction of the lateral DOF reduces the damping ratio of the torsional modal branch, which in turn reduces the stability of the system. Furthermore, among the terms constituting the damping ratio of the torsional modal branch, the torsional-vertical coupled aerodynamic damping ratio and the torsional-lateral coupled aerodynamic damping ratio are both not conducive to the stability of the torsional modal motion.