The consistency of hydrological sequences was lost due to climate change and human activities. To conduct a recheck analysis on extreme flood control risk using inconsistent hydrological sequences, we selected as the study area the watershed of the Xidayang reservoir that was subjected to significant changes in land surface conditions, and applied the hydrological variation diagnosis system to an annual maximum peak flow series of this reservoir to detect its change trend, change point, and type of change. Then, a frequency analysis method for inconsistent hydrological series was developed to calculate this peak flow series for the past and current land surface conditions, and hence the corresponding design peak flows of different frequencies was obtained. Finally, we took the check flood level of the reservoir as the control index on its extreme flood control risk and estimated this extreme risk using the frequency analysis method. The calculations reveal that for the past and current land surface conditions the extreme risks were 0.008% and 0.005%, respectively. It is obvious that these rechecked extreme risk values are lower than the check flood criterion, i.e. a 10000-year flood (0.01%). The results provide technical guidance for the Xidayang reservoir with respect to its dynamic flood level control, full utilization of flood resource in flood season, and improvement of comprehensive benefits.