Abstract: When the downstream stilling basin of the sluice flows freely at the end sill, the downstream river bed water depth ht is often less than the corresponding critical water depth hk,and the hydraulic calculation of stilling basin cannot meet the requirements of the hydraulic design of relevant sluice design specifications. At present, the hydraulic calculation method of the stilling basin with free discharge is not perfect, which brings great inconvenience to the engineering design. Based on the summarizing of the research results of the downstream stilling basin of the sluice, and under the assumption that the downstream river bed depth ht of the stilling basin with free discharge is equal to the critical depth hk,the calculation method of the tail height of the stilling basin with free discharge is discussed,the corresponding calculation method is put forward. The results are verified by the hydraulic model test, which has the characteristics of convenient calculation, and can be used as a reference for similar engineering design and operation.