Extreme risk can withstand faced flood control level of the reservoir is unpredictable，in this paper，the analysis method of the commonly used dam site flood discharge frequency(empirical frequency calculation and P-Ⅲ frequency curve suitable wire alignment)combined with the flood regulating calculation，the limit risk ratio of the flood water level dynamic control is obtained.On the one hand，the limit risk ratio of a certain flood water level in the reservoir can be understood in time，Thus the maximum flood standards with the reservoir can withstand.On the other hand，through the query curve diagram，the threshold risk ratio of the known reservoir，the water level of the reservoir under the limit control index can be deduced.