Abstract: Based on the water quality and flow monitoring data from 2016 to 2024 at key sections, including the “Five Rivers and Seven Inlets” entering Poyang Lake, this study analyzed the total phosphorus (TP) inflow flux and its temporal patterns into the lake. Four classic models for calculating lake water environmental capacity—the Vollenweider, Dillon, Goda, and OECD models—were applied to estimate the TP environmental capacity of Poyang Lake. By comparing the outputs, the average value from the Dillon, Goda, and OECD models was selected as the theoretical environmental capacity. The results indicated that the annual TP inflow to Poyang Lake fluctuated between 7,619.85 and 19,183.91 tons and correlated significantly with inflow volume, indicating that hydrological conditions were the primary driver. Significant discrepancies (with coefficients of variation exceeding 40% in some years) were observed in the TP water environmental capacity calculated by the four models. The Vollenweider model yielded the lowest results with the greatest relative deviation (−31.12%). In contrast, the results from the OECD, Goda, and Dillon models were within an acceptable range of difference (CV < 10% for most years) and followed similar trends. Using their mean value as the theoretical total phosphorus water environmental capacity, the interannual variation ranged from 6,340.03 to 12,065.24 t/a. Poyang Lake's annual TP water environmental capacity was thus overloaded by an average of 33.59% during the study period. Specifically, the TP inflow from both the Gan River and Rao River exceeded their allocated limits, 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 average surplus of 1.23%. Based on the spatiotemporal characteristics of TP inputs and the current water environmental capacity, this study analyzed its influencing factors, considering hydrological conditions, external pollution loads, internal lake processes, and climate change. Corresponding measures for enhancing the environmental capacity were proposed to aid in managing phosphorus pollution in the Poyang Lake basin. The findings offer a reference for applying multi-model comparisons in lake water environmental capacity assessment and management.