Since the navigation environment is complex when large fleets passing the No.4 navigable bridge opening of Wuhan Yangtze River bridge during low-water season, a waterway regulation engineering is going to be performed. The numerical simulation of flow field is simulated and analyzed before and after the waterway regulation engineering, and the results are applied to computational analysis the width of actual navigable waters in the No.4 navigable bridge opening. Based on the numerical computation results, the waterway conditions, aperture of bridge piers, and results of flume tests, the width of actual navigable waters are calculated. Compared with the requirements of one way single navigation span in the instructions of the Navigation Standard of Inland Waterway (GB 50139—2004), the research results show that the flow condition and the width of actual navigable waters are adequate.