In order to study the amplitude effects on self-excited aerodynamic forces of the bridge main deck section，a thin plate section was taken as the research object，and the amplitude effects of aerodynamic self-excited forces were studied by using computational fluid dynamics（CFD） method. First，the numerical simulation method of forced vibration was used to investigate the flutter derivatives and aerodynamic hysteresis phases of the thin plate section under different amplitudes. The aerodynamic spectrum characteristics of the thin plate section were analyzed. Then，the numerical simulation method of free vibration was used to study the flutter response evolution characteristics of the thin plate section. The numerical simulation results of forced vibration for the thin plate section show that the torsional amplitude has significant effects on the flutter derivatives of the thin plate section. At high reduced wind speed，the flutter derivative A*2 changes from negative to positive as the amplitude increases. The effect of vertical amplitude on the flutter derivatives of the thin plate section is relatively small. As the torsional amplitude increases，the sinusoidal value of aerodynamic hysteresis phase of the thin plate section changes significantly. When the torsional amplitude is larger than eight degrees，there are obvious high-order harmonic components in the aerodynamic forces of the thin plate section，mainly for the third and fifth order components. The high-order components of aerodynamic forces of the thin plate section caused by the vertical amplitude are not obvious. The numerical simulation results of the free vibration of the thin plate section show that the instantaneous frequency，damping ratio and the phase difference between the vertical displacement and torsional displacement change with the vibration amplitude during the process of flutter divergence.