Phosphorus is one of the most common exceeding standard factors in lakes of China when environmental evaluation. Lake Taihu is symbolic lake in China for lake eutrophication control. In 2024, total phosphorus concentration (TP) in Lake Taihu achieve to standard III request, in other words, TP was lower than 0.05 mg/L. To understand the reasons for TP reduction in Lake Taihu, long-term temporal and spatial variations of phosphorus were analyzed by monitoring data of Taihu Laboratory for Lake Ecosystem Research, Chinese Academy of Sciences during 2005 to 2024. It was found that both temporal and spatial variation of phosphorus in Lake Taihu were large. While nitrogen concentrations in Lake Taihu were continue decreased. Since 2022, yearly whole lake total nitrogen concentration was lower than 1.60 mg/L. And the cyanobacterial bloom intensity and phosphorus concentrations were significantly decreased simultaneously. TP concentration in Lake Taihu showed a weak positive relationship with inflow TP loading. During the significantly decrease of TP in lake, the external TP loading was lower than 0.55 gP/(m2•yr). However, external TP loading should lower than 0.32 gP/(m2•yr) if it posing significant influence on TP concentration in lake water. Therefore, co-limitation of nitrogen for cyanobacterial bloom must be one of the key factors of significant reduction of TP concentration in water of Lake Taihu in the past 3 years. The research suggested that reduction external TP loading lower than 0.55 gP/(m2•yr), and synergistic reduction of nitrogen is the feasible strategy to achieve the restoration target in large shallow subtropical lakes.