Abstract: The water flow energy of high-head lock is huge, and the hydraulic indexes are quite high, which puts forward higher requirements for the layout and type selection of the filling and emptying system.The maximum design head of Datengxia single-step lock is 40.25 m. The second diversion port of the filling and emptying system is intended to adopt a self-diffluence structure and there is no precedent at home and abroad. A systematic research on the diversion port and the performance of filling and emptying system is carried out in this study by means of three-dimensional numerical simulation. The hydrodynamic characteristics of the second diversion port at typical charging and discharging time are analyzed in detail. What's more, the feasibility of its application in practical engineering is demonstrated. The results show that the diffluence effect is relatively good in the process of charging and discharging through reasonable arrangement of the outlet hole size of each branch corridor. However, the typical flow velocity and pressure distribution area will appear inside the diversion port, especially in the case of discharging. There is a large region of triangular low velocity turbulent flow near the big pier head, which may have an adverse impact on the structure of the diversion port. The research results can provide reliable technical reference for fully understanding the hydraulic characteristics of this type of diversion port and also contribute to its further improvement and optimization.