Flow discharge compensating operation in cascade reservoirs can bring generating benefits of cascade reservoirs in definite conditions. But the upstream reservoir may suffer dual losses of water quantity and water head if the precision of inflow forecast fails to meet the requirement. Therefore risk assessment in compensating operation is an important task for decision-making. As the precisions of inflow forecast in compensation period and refill period are two risky factors which may result in risk generating， the risk generated in compensating operation and the probability of inflow forecast are illustrated within the introducing of risky factors. The method for calculating the water level in the end of compensation period and refill period is proposed. The calculations of the risk generated in compensation period and refill period and combinational risk in the two periods are established， and the decision flow diagram of discharge compensation is conducted based on the expected refill water level and certain acceptable risk. The result of application case shows that the model developed is effective in decision-making.