In order to reveal dynamics changes in phosphorus release from the sediments of wetlands in a mid-temperature steppe without external phosphorus input, the relationship between phosphorus content in the overlying water, sediment, and interstitial water was studied using the variables control method. The results showed that, during the incubation period, the content of total phosphorus in the overlying water and the content of total phosphorus in interstitial water both presented in the order of August > September > July. Furthermore, the correlation relationships between phosphorus content in the overlying water, total phosphorus in the interstitial water, and the ratios of sediment Olsen-P to total phosphorus in the interstitial water, indicated that Olsen-P might be a kind of release form of phosphorus - from the sediment to the overlying water of the wetlands in Hulunbeier steppe, without external phosphorus input.