Abstract: To understand the effect of phosphorus released from sediments to water of the semi-enclosed bay, the characteristics of phosphorus forms, critical influences and the temperal-spatial differences of phosphorus releasing potential were studied in the surface sediments of Daya bay. The results showed that the average concentration of total phosphorus (TP) in sediments was 429.65×10^-6. Inorganic phosphorus (IP) and organic phosphorus (OP) were 286.57×10^-6 and 143.08×10^-6, respectively. IP accounted for 67.83% of TP. Acid-extracted phosphorus (HCl-P) was the main component in IP, and the average concentration of HCl-P and alkali-extractive phosphorus (NaOH-P) were 237.10×10^-6 and 56.13×10^-6, respectively. Biological effective phosphorus (OP+NaOH-P) accounted for 45.63% of TP. There was a high releasing potential of sediments phosphorus in Daya bay. In addition, there was a clear temperal-spatial distribution pattern that the releasing potential of phosphorus was higher in summer than that in winter and, higher in near-shore than that in offshore or bay mouth. The grain size, Eh, content of organic matter (especially those from phytoplankton blooms) had an important influence on releasing potential of sediment phosphorus.