Because of the difference of flood characteristics in Mainstream and its tributaries of Ningxia section of the Yellow River,the flood volume and the complex combinations,and the severe flood control situation of Qingtongxia reservoir during flood season,so it is necessary to analyze the sources and combinations of Qingtongxia reservoir inflow floods.The paper built the joint distribution of flood variation in mainstream and its tributaries based on Copula function, and then calculated the composition of the flood elements which were most likely happen in mainstream and its tributaries by the method of the most likely composition. Results show that the design flood in Qingtongxia reservoir section is mostly from the mainstream in upstream section of the Yellow River.It is necessary to consider the influence of tributary on Qingtongxia reservoir section for reasonable flood management.Through comparing the method of the equivalent frequency composition and the measured flood, the result indicated that the situation of flood composition and flood hydrograph derived by method of the most likely composition is more reasonable. So we can gain the reasonable influence by the mainstream and its tributaries to Qingtongxia reservoir, The method can provide a more reasonable scheme of upstream flood composition for Qingtongxia reservoir section.