Abstract: The decaying process of floating Ulva prolifera was simulated in laboratory to study laws of the nutrient release.Results showed that the weight and chlorophyll content of U.prolifera had a declining trend, respectively.Floating and cultured U.prolifera weight decreased to 0.46 g and 0.79 g at 12 d, respectively.The chlorophyll content of floating and cultured U.proliferaalso decreased to 0.64 mg/g and 0.99 mg/g at 12 d.The phosphorus and total inorganic nitrogen (DIN) concentrations in the experimental group had an upward trend after the first drop.Besides, increased amount of phosphorus and DIN concentrations in the floating U.prolifera experimental group was higher than the cultured U.prolifera group.At 12 d, the phosphorus and DIN concentrations in the floating U.prolifera experimental group reached to 3.95 μmol/L and 76.55 μmol/L.Ammonia nitrogen was released to water from 6th day in the floating U.prolifera experimental group, leading to the rise of DIN.The decay of a large amount of floating U.prolifera resulted in oxygen deficiency in water and released large amounts of nutrients, leading to deterioration of water quality.Therefore, floating U.prolifera should be cleaned up quickly.