In view of the limitation of the code in taking the value of the double-deck girder drag force coefficient for large-span truss bridges， a method of calculating the drag force coefficient of the girder is proposed. Based on the wind tunnel test results of the bridge’s construction phase segmental model， its aerostatic coefficients were calculated using CFD after simplifying the model of the double-deck truss girder. The effect of characteristic size on the drag force coefficient of the girder was analyzed using different windward face characteristic sizes， the number of truss bays， and the truss spacing. The influence trend of different truss spacing and number of bays on the drag force coefficient of the girder is discussed. The results show that the characteristic height and characteristic width of the windward face have different effects on the drag force coefficient of the girder. Therefore， the fitting formula of the drag force coefficient of the girder of a double-deck truss is proposed， which takes the longitudinal height ratio of the truss as the variable. When the truss spacing is small， the number of trusses has no effect on the drag force coefficient of the girder. However， when the ratio of truss spacing to truss height reaches 3， the change in the number of trusses has a considerable impact on the drag force coefficient of the girder.