Lake Poyang is the largest freshwater lake in China and an important wetland ecosystem with international protection significance. Affected by climate change and anthropogenic activities, the hydrological rhythm of Lake Poyang has changed significantly, which seriously threatens the function and stability of the wetland ecosystem. Shallow lakes and rivers are the main natural units in Lake Poyang wetland, which store abundant environmental change information and directly affect the biogeochemical processes of wetlands. In order to further understand the regional hydrological process, a systematic collection of precipitation, river water and main shallow lake water in the Lake Poyang National Nature Reserve was carried out from January to December 2019, and the stable isotopes of shallow lake-river water system were comprehensively analysed for their dynamic change characteristics and indicative significance. Results showed that the isotopic composition of Xiuhe River and Ganjiang River had obvious seasonal variations, which was the most enriched in April, gradually depleted from May to July, and then showed a trend of continuous enrichment, similar to the temporal variation of precipitation isotopes. Spatially, the isotopic composition of each section in Xiuhe River and Ganjiang River had a relatively stable distribution along the distance. The isotopic composition of the lower section of Ganjiang River did not show significant changes in most months after the inflow of Xiuhe River, possibly related to the large discharge difference between Xiuhe River and Ganjiang River. The annual variation range of stable isotopes in shallow lake water was larger than that in river water and was relatively more enriched. The δ²H-δ¹⁸O relationships of the shallow lakes were close to the local meteoric water line, and had smaller evaporation line slopes and systematic deviation, indicating that the shallow lakes were mainly recharged by local precipitation and had experienced a certain degree of evaporation. The mean residence time (MRT) of Xiuhe River and Ganjiang River was 1.54 years and 0.81 years respectively based on the exponential flow model. The shorter MRT of Ganjiang River indicates that the Lake Poyang basin had the hydraulic condition conducive to rapid exchange of different water components, and the MRT of Xiuhe River was obviously larger due to the regulation and storage of Zhelin Reservoir upstream. Based on the assumption that the evaporation process of shallow lake water followed the Rayleigh fractionation model, the isotopic monitoring revealed that the average annual evaporation loss of Lake Banghu, Lake Shahu and Lake Dahuchi was 15.5%, 15.0% and 14.1%, respectively. The difference of evaporation degree among different shallow lakes could be associated with the comprehensive influence of terrain, vegetation and hydrodynamic conditions.