As the only model parameter involved in the large eddy simulation（LES），the empirical Smagorinsky constant（CS）is dependent on special flow condition. In order to provide acceptable value of CS to investigate aerodynamic characteristics of the flat box girder using the LES，multiple cases are carried using CS ranged from 0.032 to 0.7，as well as using the dynamic subgrid vis cous model. The results are compared to that of the wind tunnel tests，and the variation of aerodynamic characteristics with CS is al so analyzed. It is found that the mean surface pressure and mean aerodynamic forces，which are in agreement with wind tunnel test，are not sensitive to the variation of CS. However，the variation of CS imposes significant effects on the RMS of mean surface pres sure and aerodynamic forces，which show clear difference with the wind tunnel tests. The increase of CS reduces the fluctuation of aerodynamic forces，or even flats their time histories. In addition，the LES using the dynamic subgrid viscous model cannot provide reasonable estimation of vortex shedding and surface pressure of box girder. For CS ranged from 0.064 to 0.27，the vortex shedding St number characterized by a single peak frequency estimated by the LES agrees well with wind tunnel results. As regards to cap ture unsteadiness of turbulent flow，a lower value among the ranged CS is suggested. It is concluded that the increase of CS will in crease subgird turbulent viscosity，resulting in enhanced flow dissipation，which will in turn reduce LES capability to capture un steady feature in turbulent flow. Consequently，the fluctuation of simulated aerodynamic forces will decrease.