In agricultural watersheds across China, small water bodies serve as important phosphorus retention and storage sites, acting as protective barriers for rivers and lakes. However, under conditions of long-term phosphorus accumulation and specific environmental factors, there is a risk of phosphorus release. Currently, there is a lack of research on the phosphorus release risk of small water bodies and on the dynamic changes in phosphorus retention and release, as well as the influencing factors, through long-term experiments. This study focuses on a typical agricultural watershed and conducts sediment sampling from 4 types of ponds, along with a 6-month static incubation experiment. The study analyzes the phosphorus forms in sediments and the phosphorus exchange dynamics at the sediment-water interface, assessing the phosphorus release risk of the ponds and its influencing factors. The results showed that: (1) The total phosphorus (TP) content in pond sediments ranged from 0.55 to 3.02 g/kg, higher than that of other types of wetlands. The highest phosphorus content was found in village ponds, while the lowest was observed in paddy ponds. (2) In agricultural watersheds of the middle and lower reaches of the Yangtze River, and under the condition of long-term existence of ponds, the phosphorus exchange at the sediment-overlying water interface exhibits significant seasonal variations: sediments release phosphorus to the overlying water in summer and autumn, while phosphorus settles from the overlying water to the sediments in autumn and winter. Given similar years of existence, the phosphorus sink capacity of ponds decreases in the following order: forest pond > paddy pond = dryland pond > village pond. This study revealed the large phosphorus accumulation in agricultural ponds in China, highlighting their long-term function as phosphorus sinks that transform into phosphorus sources during the summer and autumn. The findings also clarified the influence of surrounding land use types and provided scientific guidance for improving management strategies for different pond types. These insights are widely applicable to non-point source pollution control and the protection of aquatic environments in rivers and lakes.