[Background] Siertan Wetland is the largest wetland of Habahu Desert Wetland Nature Reserve in Yanchi County, Ningxia Autonomous Region. Through the annual vegetation investigation which has been carried out for 11 years, we found that the area of Siertan Wetland is shrinking year by year. Many researchers concluded that the ecotone is very sensitive to the change of environment, thus it can be used to predict the change of the ecological environment in advance. This study is to quantitatively determine the ecological ecotone boundary and the main environmental factors that affect the spatial distribution pattern of the species. [ Methods ] Firstly we used moving split-window technique to quantitatively determine the ecological ecotone. Use of the moving split-window technique required that:we chose an appropriate even number of representative samples as a whole window, then divided the whole window in half and calculated their dissimilarity coefficient. We continuously moved the window right to the next sampling point and repeated the operation above again, until the right window reached the final sampling points. We then draw a scatter diagram of the dissimilarity coefficient changes by quadrats. Further, according to the result of determination we divided the research area into wetland regions, ecotone and arid region, and analyzed characteristics of beta diversity in the three different habitats of the four different sample lines. Finally, Canonical Correspondence Analysis (CCA) was used to determine which environmental factors contributed most to the distribution of the vegetation. [Results] The results showed that ecotone was determined clearly by moving split-window technique under 6 - 8 quadrats, the optimal window width of east transect line was 6 quadrates, the optimal window width of west, south and north transect line was 8 quadrates. Beta diversity index showed significant differences in different habitats. The result of CCA indicated that the total salt content and moisture content were the main environmental factors that affected the vegetation distribution in Siertan Wetland. The correlation of the two environmental factors and the first species axis were respectively 0.868 4 and 0.917 0, the correlation of the two environmental factors and the first environment axis were respectively 0.841 6 and 0.891 4. [Conclusions] According to the dimension of investigatory scale and the visibility of ecotone transitivity,appropriate window width can be chosen especially in the selection of appropriate smallest window width. Species with higher fitness for total salt content and moisture content variability are mainly distributed in wet region with serious soil salinization. The results of the study will provide valuable reference for Siertan Wetland’s restoration and protection. The future work of Habahu Nature Reserve administration should be focused on protecting Siertan Wetland from human being destruction, ensuring adequate water supply, improving the salinization land, and maintaining plant species diversity in wetland.