The old bridges constructed in the curved branching reach under complicated navigation environment face great risks under some water conditions. To study the effect of navigable opening adjustment on the improvement of safe navigation，we take Fengcheng Highway Bridge in Ganjiang River as an example and establish a 2D hydrodynamic numerical model of 14.7 km long to calculate and analyze the river flow characteristics and navigation flow conditions in the openings and around the bridge under several scenarios from the drought season to the flood season before and after the operation of the downstream Longtoushan Navigation-power Junction. Based on the calculated results，we demonstrate the bridge clear width scale and the feasibility of its adjustment in accordance with the code. The research results may serve as a reference for the improvement of the navigation condition of other old bridges in the curved branching river.