In order to study the effects of a group of bridges built across the Yongjiang River, a tidal river, on the flood and tide control of the river, a two-dimensional numerical model was established for the unsteady flow of the Yongjiang River based on the Delft3D model. The established model was calibrated and verified by the field observation data and used to study the influence of the group of bridges on the flow velocity and water level of the river under different flood and tide conditions. The results show that a single bridge has an effect on the flow near the bridge, while a group of bridges have a much greater accumulative influence on the flow velocity and water level and increase the influenced area. The effects of the bridges on water flow under different flood and tide conditions were compared. It was found that the effects of the bridges were more significant under the conditions of a spring tide and a small flood than under the conditions of a large flood and a neap tide.