Baiyangdian Lake is the largest freshwater wetland in North China. Fu River is one of the main rivers that flow into Baiyangdian Lake. In recent years, increasing frequency and intensity of extreme rainfall events have introduced new challenges, driving complex spatiotemporal variations in nitrogen and phosphorus concentrations and fluxes. This study systematically investigated the Fu River-Baiyangdian Lake system through field monitoring, water sampling, laboratory measurements and statistical analysis to reveal the dynamics of total nitrogen (TN) and total phosphorus (TP) concentrations and their response to extreme rainfall. Results revealed distinct seasonal trends: TN concentration was highest in spring and winter and lowest in summer, whereas the TP concentration peaked in summer and declined in spring and winter. Spatially, TN and TP concentrations decreased from upstream to downstream along the Fu River. After the "7.29" extreme rainfall event, TN concentration showed a complex increase-decrease-increase trajectory influenced by agricultural runoff, sedimentary nutrient release, and biogeochemical processes such as dilution and denitrification. TP concentration showed a simpler increase-decrease pattern driven by sediment resuspension and subsequent attenuation. Before the extreme rainfall event, nitrogen and phosphorus loads along the river gradually decreased. After the extreme rainfall event, nitrogen and phosphorus loads along the river significantly increased due to the input of non-point source pollutants. Nutrient loads surged significantly after the extreme rainfall event, increasing by factors of 10.5 to 47.4 due to the input of non-point source pollutants. These findings underscore the vulnerability of Baiyangdian Lake to extreme hydrological events and highlight the urgent need for targeted interventions to reduce pollutant loads during such events. This research provides critical insights into nutrient cycling dynamics under changing climatic and hydrological conditions, offering a scientific basis for sustainable water quality management in lake-watershed systems.