Phosphorus is the main indicator of eutrophication in Lake Taihu. In order to understand the trends and characteristics of the internal and external total phosphorus (TP) load in Lake Taihu, the main sources of TP in the lake and the reasons for the high fluctuation of TP concentration were analysed from the dynamic equilibrium of TP pollution load. The mass balance of TP load in different ways into and out of the lake was estimated and analysed based on a long series of water quality monitoring data. The results showed that the input TP load of rivers was about 1835-2799 t during 2007-2020 as the main source, accounting for 55%-73% of the TP load in Lake Taihu. The TP load transported by dry and wet deposition was 353-1380t, which accounted for about 12%-38% and was the second largest input pathway of TP load in Lake Taihu. Meanwhile, 8%-15% of the TP load was retained in the water of Lake Taihu. However, the TP discharged from the outlet river was about 516-906 t, accounting for about 13%-30% of the TP load in Lake Taihu; the TP removal by harvesting aquatic animals and plants was 115-312 t, accounting for 4%-12% of the TP load in Lake Taihu; and the percentage of TP output by drinking water extraction was about 2%-3%. Therefore, about 41%-74% of the TP load remained in the lake, which is an important endogenous factor influencing the epilimnetic TP concentration in Lake Taihu. In addition, the increase in temperature and water flow velocity in Lake Taihu had both accelerated the release of TP from sediment into water, resulting in sediment being one of the limiting factors for TP control.