AbstractOptimal operation under uncertainty is a classical, albeit challenging, issue in reservoir operation. This study focused on the flexibility to tackle the inflow uncertainty of reservoir operation. Using the flexibility index, the inflow can be described as an adjustable interval range rather than a probability distribution, and all constraints should be satisfied within this interval. The flexible operation (FO) model is proposed by considering the flexibility index and hydropower generation simultaneously. If reservoir levels at starting and ending are fixed, the optimal hydropower output first increases and then decreases as the inflow increases. Consequently, the FO model can be solved by inputting either upper or lower boundary inflows. China’s Three Gorges Reservoir was considered as a case study. The results revealed that a trade-off between hydropower generation and the flexibility index exists. The reservoir water levels were lowered in the nonflood seasons to improve flexibility. The FO model can tackle uncertainty effectively, although it is slightly conservative in terms of hydropower generation. The proposed flexible method can be helpful in reservoir operation under uncertainty.