River young water fraction and residence time are critical parameters for describing streamflow processes, as well as the transport of pollutants and nutrients within a watershed. Determining the sources of water and its residence time within the watershed is critical for us to understand watershed behavior and functionality. To quantify the river young water fraction and residence time for the Lake Poyang Basin, this study conducted high-frequency monitoring of stable isotopes (δ¹⁸O and δD) in precipitation and river water for five major inflow rivers of the basin, i.e., the Gan River, Xiu River, Xin River, Fu River, and Rao River (hereinafter refered to as “Five Rivers”). The study analyzed the variations in δ¹⁸O and δD values for precipitation and river water using a sine fitting method. The study achieved the following findings. (1) The deuterium and oxygen isotopes in the “Five Rivers” water and precipitation exhibited similar characteristics, i.e., low isotopic values during the rainy season (May to July) and high values during the dry season (December to February in the following year). There was a high correlation between river water isotopic variations and precipitation isotopes as well as groundwater isotopes, indicating good hydrological connectivity among different water bodies in the watershed. (2) The young water fraction in the “Five Rivers” catchments, with an age less than 2 months had a range from 3% to 42%. Its mean residence time varied from 123.83 days to 2495.0 days. Among the inflow rivers, the Gan River had the longest mean residence time and the lowest young water fraction, while the Le'an River and the Chang River exhibited the highest young water fraction and the shortest mean residence time. (3) Catchment area and the main stream length per unit area were identified as the primary driving factors for the young water fraction in the “Five Rivers” catchments. Slope was negatively correlated with the young water fraction. However, the negative correlation was overshadowed by the positive correlation between the main stream length per unit area and the young water fraction. (4) Compared with other catchments, the Xiu River and Gan River catchments were more significantly influenced by evaporative fractionation. (5) Lake Poyang water had significantly higher isotopic values than river water. Within Lake Poyang, the Hukou station had lower isotopic values than the Duchang station. This result indicated a combined influence of Yangtze River and evaporative fractionation on Lake Poyang water. The study improved our understanding of hydrological connections and processes within the Lake Poyang Basin, and thus provided a scientific reference for modelling mass transport and learning the underlying mechanisms.