The analysis of flood region composition is the basis for the design flood estimation of downstream cross sections affected by the regulation of the cascade reservoirs. One of the methods recommended by "Regulation for calculating design flood of water resources and hydropower projects" for calculating the flood region composition is the discrete summation method. Before using this method, it is necessary to perform independent test and transformation on the variables in each division. However, the regulation does not clearly provide methods for the independent transformation in cases where there are three or more divisions. When analyzing flood region with multiple divisions, the only option is to perform discrete summation in a hierarchical manner. Based on the independent transformation method by LDLT decomposition of the covariance matrix, this paper proposed the independent transformation discrete summation method based on LDLT decomposition, which can directly perform independent transformation on multiple composition variables and then perform discrete summation. As a case study, this paper analyzed and compared the design flood results obtained from the hierarchical discrete summation method, the improved discrete summation method based on Copula functions, and the independent transformation discrete summation method based on LDLT decomposition at the downstream cross section of cascade reservoirs in the Yalong River Basin. The results show that the design flood volume reduction rates calculated by the three methods are basically close to each other. The independent transformation discrete summation method based on LDLT decomposition proposed can provide comparative option for the design flood calculation of downstream cross sections affected by cascade reservoirs.