Abstract: A large-eddy numerical model was used to simulate flow in a rectangular open channel blocked with a bridge pier or multiple bridge piers, focusing on effects of the bridge piers upon the flow characteristics and turbulent structures. Calculations were performed for six values of pier spacings s/D with same Froude numbers. The calculated results were compared well with previous laboratory results. And the variations in velocities, water levels in front of the central bridge pier and behind it and the vorticity were deeply analyzed based on the calculated results. Numerical simulation results show that for the closely spaced conditions (s/D=2), the absolute values of the velocity in vertical direction |Uz| in the region (0＞x/D＞-5) were larger than the one in the same region for other conditions. On the contrary, the velocity in vertical direction Uz in the region (x/D< 5) was almost zero and the bridge piers hardly affected the flow field; for the closely spaced conditions (s/D=2), the values of the hydraulic drop upstream and downstream of the central bridge pier were to be the maximum due to the duplicate effects given by the bridge piers; and for the widely spaced conditions (s/D＞5), the effects caused by the bridge piers on both sides of the central pier in the wake area downstream of the central bridge pier became weaker and the duplicate effects given by the bridge piers could be ignored. The lengths of the vortices corresponding to the dominant frequency at different locations behind the central bridge pier were studied based on the spectral analysis and calculated vorticity. The length of the vortices at x=0.5D was equal to a pier diameter (D), so that the vortices at x=0.5D behind the central bridge pier were mainly caused by the wake downstream of a bridge pier; and the length of the vortices at x=5D was equal to three times of the pier diameter (3D), so the vortices at x=5D were caused by the duplicate effects given by the multiple bridge piers.