In order to determine the effects of herbaceous plants on the release process of soil nitrogen and phosphorus, a simulation experiment of reservoir water level fluctuation was conducted under Cynodon dactylon and Alternanthera philoxeroides. The results showed that:(1) After the end of flooding (32 days), the soil total nitrogen (TN) and total phosphorus (TP) decreased by 11.75% and 25.28%, respectively in Alternanthera philoxeroides, and by 3.62% and 25.77%, respectively in Cynodon dactylon. (2) The dry-wet alternative environment mainly affected the changes of ammonium nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N) and available phosphorus in the soil, and had little effect on TN and TP contents in the soil. (3) The death of Cynodon dactylon increased the amount of soil TN, NH₄⁺-N and TP, indicating that the inundated vegetation filter belt could purify N, P and other pollutants in the runoff. However that part of nutrients absorbed by the plants would in turn release to water system by the decomposition of plant biomass. The decomposition of the plant biomass once again entered into the water system or the soil, and thus the purpose of effectively preventing and controlling agricultural non-point source pollution could not be achieved. This study provides a theoretical basis for the using of vegetation buffer zone to prevent and control the eutrophication of water system in the Danjiangkou Reservoir.