Abstract: Reverse flow has an important impact on hydrodynamic changes and water environment protection in Poyang Lake Basin. A correct understanding of the causes and conditions is of significance to clarify the function mechanism of reverse flow and study the intensity and coverage of influence of reverse flow. From the perspective of unsteady flow flood wave propagation, by analyzing the causes of reverse flow, and considering the characteristics of reverse flow when Poyang Lake is in fluviatile facies and in lacustrine facies, this paper puts forward the Yangtze River action intensity index and Poyang Lake action intensity index. On such basis, the comprehensive river-lake action intensity function is introduced. With the measured data, this paper has analyzed and studied the relational expression of the comprehensive river-lake action intensity function, as well as the occurrence conditions and quantization index for reverse flow. It is verified that the proposed reverse flow quantization index can be used to distinguish river-lake uplift and reverse flow phenomena, and predict reverse flow when Poyang Lake is either in fluviatile facies or in lacustrine facies, with improved accuracy rate in predicting the starting time, total duration and annual occurrence times of reverse flow. The results can be used to study river-lake relations and the actual engineering for comprehensive utilization of water resources in Poyang Lake Basin.