The fluctuating backwater area of cascade hydropower station is affected by the discharge of upstream hydropower station and the water level in front of downstream dam,thus the determination of the designed lowest navigable water level is very complicated.Traditionally,the guaranteed rate of designed lowest navigable water level mainly carries on the statistics to the discharge,but the index is not comprehensive enough;moreover,there is certain contingency in the selection of the cases combination for the calculation of designed lowest navigable water level,and the operability is not strong in the actual design process.This paper presents a method for calculating the combined guaranteed rate of inflow volume and water level in front of the dam,which takes into account the probability of the combination of inflow volume and water level in front of the dam.By using this method,the designed lowest navigable water level in the variable backwater area of cascade hydropower stations can be reasonably determined.At the same time,combined with the dispatching scheme of the power station,the channel could be regulated with the least investment in order to achieve the optimal effect.