The Huangtangqiao Hydro-junction of Changshan River of upstream and the middle reaches of Qiantang River is located in the narrow section of the mountain stream.There are unfavorable flow regimes such as lateral-flow and backflow in the entrance area and the connecting section,making it difficult to meet the navigation requirements.The flow conditions of low-head and high-discharge junction set up on the narrow river channels of mountain streams face greater tests and have less optimized space.The causes of the reduction in discharge capacity and the adverse flow conditions of the downstream channel are analyzed by the 1:80 overall physical model test.On the basis of engineering measures such as moving the discharge lock and dredging the local downstream channel,the discharge capacity of the junction is effectively increased,the longitudinal flow,lateral flow and backflow velocity in the entrance area are reduced.Therefore,the navigable flow conditions in the entrance area and connection section of the downstream approach channel are greatly improved,which ensure the ships safety of passing through the ship lock.