Abstract: [Objective]The source and concentration of phosphorus are key factors determining the occurrence of eutrophication in surface water, and accurately identifying the contribution of phosphorus pollution sources across different seasons serves as an important basis for realizing differentiated control of phosphorus. [Methods]This study focuses on Tangxun Lake (a typical urban lake) as the study area. It comprehensively adopts methods including field observation, phosphate oxygen isotope composition analysis, and MixSIAR model simulation to accurately analyze phosphorus pollution sources in different seasons and their contribution to dissolved inorganic phosphorus (DIP) in lake water. [Results]Results showed that during the wet season, urban stormwater sewage contributed the most to DIP in lake water, accounting for 35.8%, followed by domestic sewage (18.9%) and fishpond water (15.9%). In the dry season, the contribution of urban stormwater sewage to DIP was 30.7%, while that of domestic sewage was 21.4% and fishpond water was 18.7%. Overall, urban stormwater sewage had the highest contribution to DIP in lake water, but there were significant seasonal differences. These differences were mainly caused by the combination of excessive summer precipitation and strong microbial degradation. Several unknown phosphorus sources form a stable contribution system, ultimately resulting in no significant seasonal variation in the contribution of unknown sources to DIP in lake water. [Conclusion]This study clarifies the seasonal contribution characteristics of phosphorus sources in typical urban lake with complex pollution, and provides a scientific basis and technical reference for the accurate source identification and targeted control of phosphorus pollution in similar lakes.