Abstract: Based on the water quality and volume monitoring data at key sections, including the "Five Rivers and Seven Inlets" entering Poyang Lake, from 2016 to 2024, this study analyzed the total phosphorus (TP) inflow flux and its variation patterns and characteristics. Four classic models for calculating lake water environmental capacity, including the Vollenweider, Dillon, Goda, and OECD models, were applied to estimate the water environmental capacity of TP for Poyang Lake. By comparing the outputs, the average value calculated by the Dillon, Goda, and OECD models was adopted as the theoretical water environmental capacity, based on which the TP water environmental capacities allocated to the Five Rivers by runoff proportion were calculated and then compared with the actual inflow flux. The results showed that: 1) From 2016 to 2024, the annual TP inflow flux to Poyang Lake fluctuated between 7 619.85 and 19 183.91 t/a, which was significantly positively correlated with inflow volume. This indicated that hydrological conditions were the primary driver. 2) Significant discrepancies were observed in the water environmental capacity of TP calculated by the four models. The Vollenweider model produced the lowest values and exhibited the largest relative deviation (−31.12%). In contrast, the results from the OECD, Goda, and Dillon models were within an acceptable range of difference (with coefficients of variation below 10% for most years), and their results followed similar trends. Their mean value was used as the theoretical water environmental capacity of TP for Poyang Lake, and the interannual variation ranged from 6 340.03 to 12 065.24 t/a. 3) From 2016 to 2024, the annual water environmental capacity of TP for Poyang Lake was overloaded by an average of 33.59%. Among the five major inflow rivers, the Gan River and the Rao River both exceeded their allocated water environmental capacity of TP, with multi-year average overloads of 33.69% and 44.78%, respectively. The Xin River and Fu River showed a surplus only in 2022 and 2020, respectively, resulting in long-term average overloads of 34.19% and 54.57%. In contrast, the Xiushui River maintained a long-term surplus of 1.23% on average. 4) Generally, the water environmental capacity of TP for Poyang Lake was primarily influenced by hydrological conditions, external pollution loads, internal lake processes, and climate change. Enhancing the water environmental capacity for Poyang Lake hinges on further reducing total phosphorus load from the Gan River and other sources. It was recommended to implement comprehensive measures integrating basin water resource management, ecological regulation, internal source control, and enhanced early warning capabilities for extreme climate events.