The phosphorus pollution of the sediment from Dongxin River, an urban river in Hangzhou, was surveyed. Phosphorus release under various flow velocity (0.002-0.02 m x s(-1)), quality of overlying water (the total phosphorus concentration: 0.18, 0.072 and 0.007 mg x L(-1)), depth of overlying water (5, 10 cm) were carefully studied in a hydraulic simulation setup. The results indicate that phosphorus was feasible to release from the sediment with high ratio of water content (40.09%-68.57%) under alkali condition (pH 7.2-8.3). The phosphorus release was also influenced by high content of phosphorus (1.04-2.51 g x kg(-1)) in the sediment. Phosphorus release was mainly in the form of suspended solid under dynamic hydraulic condition, and depended definitely on flow velocity. The release rate reached 147.36 g x (m2 x h)(-1), which approached the maximum value, at flow velocity 0.008 m x s(-1) (i.e. 0.05 m x s(-1) in archetype). Phosphorus release, especially the initial release rate, was obviously affected by the quality of overlying water. Phosphorus tended to release under river water than tap water. However, the depth of overlying water had little influence on phosphorus release in shallow water. Therefore, it is possible to restrain or enhance phosphorus release by adjusting the hydraulic conditions and the quality of overlying water.