Abstract: The paper systematically discusses the relations of uncretainty between the current velocity head,design head of erosion protection of the downstream slope blocks,construction flood and sizes of diversion structure during operation of rockfill cofferdam,and analyzes the operational safety affecting cofferdam slopes.Based on the computational method for the worst flood discharge during overtopped rockfill cofferdam,a design overflow risk is measured by the probability that current velocity head of cofferdam downstream slope blacks exceeds its design erosion protection head.A computational method for design risk in overflow period of cofferdam is given by using the JC method of reliability theory,providing a theoretical rule of structural design,energy dissipation and erosion protection for overtopped rockfill cofferdam.