Abstract: In order to investigate the effects of motion phase angle on the aerodynamic self-excited forces, in the report, the computational fluid dynamics simulation was performed, the motion amplitudes and phase angles were changed, the nonlinear aerodynamic self-excited force properties of a thin plate section were investigated. The results indicated that in the cases of small motion amplitudes, the self-excited aerodynamic loads were irrelevant to motion phase angles, which is in agreement with the classic aerodynamic theory of streamlined sections. With the increase of the amplitude, the nonlinear properties of the self-excited forces displayed in two aspects. First, the new high-order load components arose, however, when the motion amplitude remained stable, the motion phase angle changes did not bring the new higher order component of self-excitation force; the change of the phase angle resulted in the change of the self-excited force components, and which affected the proportion of each order component of self-excitation force significantly. In the cases of small motion amplitudes, the effects of motion phase angle on the flutter derivatives were negligible. With the increase of the amplitude, the flutter derivatives A_1^* , A_2^* , A_4^* , and H_4^* become sensitive to phase angles, while A_3^* , H_1^* , H_2^* , and H_3^* remain insensitive. These properties suggested that because of the dependence of the self-excited aerodynamic loads on the motion phase angle, for the streamlined section experiencing large motion states, the traditional flutter derivative aerodynamic description become invalid.