Abstract: The double-line locks of Changsha navigation-hydropower junction share an approach channel， in which forms unsteady flow during locks empting and filling water because of large discharge flow. The unsteady flow is very complex， so ship navigation and berthing safety may subject to dangers. To ensure the safety of the ship navigation and berthing， hydraulic characteristics of complex unsteady flow in the approach channel under the most unfavorable operation condition are studied in detail by means of a three dimensional mathematical model. The formation， developing and spreading mechanisms of wave are analyzed， and these hydraulic factors such as wave height， wave velocity， gradient and mooring force are obtained from numerical simulation. And some improvement measures to help the double locks to operate by means of staggered flow peak and the locks to discharge the water not to the downstream navigation channel directly but to the side pipeline are proposed. These measures mentioned above have improved effectively the flow condition in the approach channel and been applied successfully to the supporting projects. Research results can be taken as a reference for the similar projects in future..