Abstract: The catchment area of basin Karst above the dam site of Zhaojiahe reservoir is composed of open flow area and closed flow area, among which the closed flow area is mainly distributed in the form of low-lying land and underground river. The design flood of Zhaojiahe reservoir dam site is calculated by the method of rain and flood, and the hydrological analogy respectively under natural conditions and engineering measures. After comprehensive analysis of the calculation results, the method of rain and flood is recommended to calculate the design flood according to the whole area.