In order to investigate the characteristics of nonlinear aerodynamic system of a thin plate, flow field around the thin plate under forced unit impulse excitation was simulated by the unsteady Reynolds-averaged Navier-Stokes (RANS) equations and SST k-ω turbulent model, and time histories of aerodynamic force were obtained. The nonlinear aerodynamic system of the thin plate was then identified based on the Volterra theory. The investigation indicates that the present aerodynamic model can provide reasonable output under the excitation within the defined ranges of frequency band and amplitude. Moreover, under the present range of forced frequency and amplitude, no clear dependence of model response on the frequency and amplitude is observed. It is also found that the aerodynamic nonlinearity of the thin plate is not significant, and the flow around the thin plate can be considered as an aerodynamic system with weak nonlinearity. This study confirms that the CFD method is of remarkable advantage for the identification of aerodynamic system of bridge girders.