Abstract: On October 10, 2021, the Danjiangkou Reservoir experienced a once-in-20-year major flood, with the water level reaching the normal storage level of 170 meters for the first time. During the storage period, the total phosphorus concentration (TPC) of the reservoir increased obviously. It had a certain impact on the water supply safety of the middle route of South-to-North Water Diversion Project. The hydrological and water quality monitoring data of Danjiangkou Reservoir and inflow rivers from 2020 to 2021 were collected. Based on the data, the changes of total phosphorus pollution in Danjiangkou Reservoir during flood season were compared using Spearman correlation coefficient method and base flow segmentation method. The causes of the changes were also analyzed. The results showed that during the 170m water storage period in 2021, the TPC increased by 0.5-2.9 times compared to the same period in 2020, and the area where total phosphorus exceeded the standard increased significantly. The phenomenon of total phosphorus exceeding the standard spred to the downstream of Hanku Reservoir and the upstream of Danku Reservoir. In the inflow pollution, the total phosphorus load of inflow rivers increased by 1.8 ~ 9.4 times compared with the same period in 2020, of which non-point source pollution accounted for about 90%, an increase of 16% compared with the same period in 2020. The fundamental reason for the increase in TPC during the flood season in Danjiangkou Reservoir was that the intensity of soil erosion in the water-level fluctuation zone ascended, which led to a substantial increase in the load of non-point source pollution. These results provided a theoretical basis for the prevention and control of sudden total phosphorus pollution caused by flood in Danjiangkou Reservoir.