Influenced by both Dahejia hydropower and Sigouxia hydropower,Dahejia to Sigouxia river reach on the Yellow River is complex in natural condition,so it is necessary to assure the design lowest navigation water level in order to suitably and economically carry out the regulations and exploitation of this river.Considering the influences of operating characteristics of two hydropowers,we establish one dimensional mathematical model to calculate the design lowest navigation water level,and obtain the reasonable design lowest navigable level.