Abstract: The operation of cascade reservoirs has significantly altered the spatiotemporal features of downstream floods, while the current flood regional composition methods do not quantitatively consider the most unfavorable or worst situation for hydraulic structure design and flood control. In this paper, we proposed the most unfavorable regional composition (MURC) method that is based on the hazard risk theory and derived by flood volume from uncontrolled downstream sub-basin and corresponding allocation probability. The case study in the Yalong River cascade reservoirs indicates that : the flood volume allocated into uncontrolled sub-basin by MURC method is always greater than the equal frequency regional composition method and the most likely regional composition method; the design flood peak, 3-day and 7-day flood volumes of the estuary section estimated by MURC method are 16 700 m³/s, 4.16 billion, and 8.51 billion m³, respectively, which are all larger than the values estimated by other regional composition methods. Theoretical derivation and statistical experiment analysis further prove that the MURC method can rationally allocate flood volume and balance the downstream flood control risk, which is worth for further study and practical application.