The impact of the Yangtze River backwater on the water surface slope of the Tuojiang river estuary is analyzed by flood levels below the Liutanba dam, flood levels of the Tuojiang river estuary and flood peak discharges of the Lijiawan hydrological station. The Longtouguan bridge once-every-three-years flood levels affected and non-affected by the Yangtze River backwater are calculated using corresponding stagedischarge relationship curves below the Liutanba dam and water surface slopes. The two cases are simulated by 2D hydrodynamic numerical model SMS to identify the navigational clearance of the Longtouguan bridge on the Tuojiang river effected by the Yangtze River backwater.