Abstract: Based on the survey of plant community of degradation swamp wetland with different enclosed years, in Hequ stud-farm. The rule of species diversity and above-ground biomass was analyzed by the species dominance indices, richness indices, shannon-wiener diversity indices and evenness indices. Results indicated that: 1) The above-ground biomass had the trend of increasing with the longer enclosed time, but got the maximum after 2 years, and then decreasing.2) With the longer enclosed time, the dominance indices dropping off, richness indices almost no change, while the shannon-wiener diversity indices and evenness indices increasing. It means that the species diversity gradually increasing.3) There was a positive linear relationship between the evenness indices and shannon-wiener diversity indices. Evenness was an important factor to increase species diversity under the premise that the number of species in a certain range.