Abstract: Evaluation on the hard-regulated and utilized volume of floodwater (HCUVF) in flood season for hyper-concentration rivers is a key issue in hydrology and water resources, but at precent the understandings of its connotations and properties and the existing evaluation methods are very limited. This work takes a case study of the Bajiazui reservoir on Jinghe River. With consideration of the operation of reservoir flood control and the sediment transport limitation on utilization of watershed water resources, and in view of water quantity and quality, HCUVF is divided into two portions, a hard-regulated volume and a hard-utilized volume. The two portions are calculated with an optimal operation model of reservoir flood control and a maximum grade approach, respectively. Results show that the hard-regulated volume much depends on the inflow flood events of reservoir. In the flood season of 1988, four such events occurred with a total volume of 4.56 million cubic meters (mcm). For the second portion, the coefficients of surplus floodwater in flood season can be determined by using the optimization results of reservoir operation and the corresponding sediment concentrations in the river flow, and its volume of 103.80 mcm was estimated. Summing up the two figures gives a total HCUVF of 108.36 mcm for the flood season of 1988. Thus, the presented method is reliable and provides an insight into development and utilization of floodwater resources and study of flood disaster risk.