Based on historical data collected over nearly 40 years and a 19-month field survey conducted from 2023 to 2024, this study investigated the long-term variation in phosphorus concentrations in the overlying water of Caohai Lake and the characteristics of phosphorus speciation during submerged vegetation recovery. The results showed that total phosphorus (TP) concentrations in Caohai Lake exhibited an overall declining trend. During the initial water~storage restoration period in the 1980s, the TP concentration was 0.461 mg/L. In 2005, when urban sewage was directly discharged into the lake, the TP concentration remained relatively high at 0.157 mg/L. After comprehensive management, TP concentrations decreased to 0.056–0.073 mg/L during 2020–2022. During the water ecological restoration period, the TP concentration further declined to 0.036 mg/L in 2024, indicating that the recovery of submerged vegetation plays an important role in maintaining low phosphorus levels in the water column.Phosphorus in Caohai Lake was mainly present in particulate form, with particulate phosphorus accounting for 54.7% of TP. Particulate phosphorus (PP) dominated in spring, summer, and autumn, with the highest proportion occurring in spring, whereas dissolved total phosphorus (DTP) dominated in winter. The TP concentration in Caohai Lake was close to that of lakes with good water quality, such as Erhai Lake, and was relatively low compared with lakes in the five major lake regions of China. Under different submerged plant biomass levels, dissolved phosphorus fractions, including DTP and soluble reactive phosphorus (SRP), showed relatively obvious differences, whereas TP, PP, and dissolved organic phosphorus (DOP) varied only slightly. Overall, reducing phosphorus input through external pollution control and restoring submerged vegetation to enhance self~purification capacity are effective approaches for maintaining low phosphorus levels in lakes. These findings provide a scientific basis for the protection, restoration, and management of Caohai Lake and similar lakes.