Resort to the topography model, the horizontal distribution characteristics of mean wind speed in a mountain valley bridge site is tested. Based on the wind-tunnel-tested flutter derivatives, the indicial functions are firstly adopted in the literature to simulate the self-excited aerodynamic forces on bridge girder sections, and, of which the simulation veracity is inspected. Then a recurrence algorithm for self-excited aerodynamic forces on bridge sections is deduced. The aerodynamic forces are indispensable in the dynamic finite element analysis,. Finally, the aerodynamic stability of a long-span valley bridge is investigated in time domain considering the influence of horizontal un-uniform distributed mean wind speed. The numerical results show that the horizontal distribution mode of mean wind speed in valley has an obvious impact on the bridge flutter threshold, which exhibits mainly in enhancing the critical flutter wind speed. In contrast, ignoring the wind horizontal distribution characteristics, as guided by the wind-Resistant code of China highway bridges, may lead to conservative results.