In order to study the effects of submerged macrophytes on different forms of phosphorus during decline phase, we adopted Vallisneria natans as the study object and measured the content of phosphorus in water and sediment. Results showed that, in water, total phosphorus (TP), dissolved total phosphorus, soluble reactive phosphorus, particles phosphorus, and dissolved organic phosphorus in V. natans changed steadily, without any significant difference compared with control groups, while all forms of phosphorus in sediment had a little rising trend. At the end of the experiment, the content of TP was 719.27 mg/kg in the V. natans sediment, and was 14.68 mg/kg higher than the initial value. The inorganic phosphorus, organic phosphorus, NaOH-P, and HCl-P contents raised for 12.06, 2.20, 7.05 and 4.29 mg/kg, respectively. The results indicated that decomposition rate of V. natans was slow and there was no significant effect of V. natans decomposition on the different forms of phosphorus between water and sediment.