Abstract: Nanhu Lake water system located in the downtown of Jiaxing City is an important part of the Hangzhou-Jiaxing water network. Based on the historical monitoring data and field investigation, the spatio-temporal distribution and morphological composition of phosphorus in water and sediment of Nanhu Lake and the surrounding river network were analyzed, and the influencing factors were discussed. The results showed that total phosphorus (TP) concentrations in Nanhu Lake were between 0.121 and 0.388 mg/L, with the high concentrations mostly near the river at the mouth of the lake, and TP concentrations in summer were significantly higher than those in other seasons. TP concentrations in the surrounding waters were between 0.026 and 0.588 mg/L, with the high concentrations in the western and northern parts. Particulate phosphorus (PP) was the major species of phosphorus in water system of Nanhu Lake. TP content in the surface sediments of Nanhu Lake and its surrounding river network ranged from 479.91 to 3 456.55 mg/kg, which was higher than that of several lakes in eastern China, and it showed a decreasing trend of TP content from the lake to the surrounding river network. Inorganic P (IP) was dominant in the sediments, of which the proportion of sum of the bioavailable IP fractions in IP contents (59.23%) was higher than the proportion of relatively stable IP fraction (40.77%). There was a significant positive correlation between phosphorus and suspended solids in water, which was affected by internal release and external inputs. According to the results, the phosphorus contamination in Nanhu Lake and the surrounding water should be controlled by decreasing the suspended matter contents in the water and implementing sediment dredging, combining with the restoration of aquatic plants.