Abstract: The underlying surface of the Hai River basin has changed significantly in recent years due to human activities. This has caused a change in infiltration, runoff and other hydrological characteristics, and thus a significant impact on the risk rate of reservoir flood control in this basin. To study such an impact in the case of the Wangkuai reservoir, we adopted a rainfall-runoff relationship method and peak-volume correlation method to modify the flood volume and flood peak series of the main flood period, and made frequency analysis of the flood series with and without the modification separately. Then the corresponding flood control risk rates were obtained for these two cases through flood regulating calculation under given different flood control levels. The results show that the underlying surface changes have caused a reduction of different degrees in various flood control risk rates of this reservoir. To ensure its flood control safety, an appropriate measure is to raise the flood control level up to 193 m for the main flood period.