With the increasing scale of wind energy being connected to the power grid, the system flexibility shows an increasingly important role in maintaining the power balance. This trend indicates that it requires a clearer understanding of how the flexibility resource provided by hydroelectric units affects the flexibility performance. It is a challenge for the flexibility assessment as one of the major reasons for the accurate quantification involves the randomness, volatility, and intermittency of wind power injected into the grid. To address this challenge, a mathematical model of a wind-hydro hybrid power system (WHHPS) is developed and verified. The flexibility indices, namely, the probability and expectation of upward/downward flexibility not supplied, are defined. Finally, the flexibility properties of different reserve ratios and line distances are quantitatively analyzed. One of the main results shows that both the upward and downward flexibility are improved with the reserve ratio increasing regardless of changes in wind speed types. The proposed methods and results provide a visualized reference for the dispatching department to evaluate the required flexibility for improving the utilization rate of wind energy.