Pettitt test and Mann-Kendall test were used to analyze the inconsistency of the annual maximum flood peak discharge sequence of the Ken Swart Reservoir, and the variant form of the sequence was determined. The consistent correction was conducted based on the theory of decomposition-synthesis, and then the annual maximum peak discharge sequence under the conditions of past and present was obtained. Taking it as the inflow process of the reservoir, the flood regulation routing was carried out according to the flood control and regulation rules, and the extreme flood risk rate of the reservoir under the two different conditions were calculated using frequency analysis method. The results show that the change point of the annual maximum peak discharge sequence occurred in 1993, whereas the overall rising trend of the sequence was not significant, and the jumping variation was the main variant form. According to the maximum water level of the reservoir under the two conditions, the check flood level(Z_d = 993. 35 m)was taken as the control index of the extreme flood control risk. The extreme risk rate of the flood control was 0. 231 23% under the past condition, while it was 0. 354 58% under the present condition. The rechecked extreme flood control risk rate of the Ken Swart Reservoir was larger than the 5 000-year check standard(0. 02%)under both conditions.