Water is an important factor that affects the plant diversity of wetland ecosystems. It is also the main driving factor for wetland plant diversity and plant growth. The study of species diversity along the water environmental gradient in the context of global change is very important. The Sanjiang Plain is China's largest concentrated freshwater wetland distribution area and one of the key areas of China's five major freshwaters and wetlands. It is an important habitat for many rare and endangered animals, a breeding ground, and a migration route to many species. However, natural wetland areas are one of the fastest changing environments due to human activities worldwide. The area of Sanjiang Plain wetland has decreased, the groundwater level reduced, and the composition of plants significantly changed due to agricultural development-oriented human activities. We studied the species composition and diversity of three typical habitats, defined as typical meadow, wet meadow, and marsh, dominated by Calamagrostis angustifolia along the soil moisture gradient in Sanjiang Plain Wetlands. The results indicated that C. angustifolia was the dominant species in the three habitats, whereas the composition of associated species was significantly differentiated between the habitats. The typical indicator species of the meadow were Lathyrus quinquenervius and Carex orthostachys. The indicator species of the wet meadow were Carex pseudoconica and C. miyabei var. maopengensis. Non-metric multidimensional scaling analysis showed that the species composition was greater in meadow, and species composition was smaller in swamps and marsh meadow compared to other types of examined habitats. With soil moisture content increasing, the advantages of Calamagrostis angustifolia decreased and species richness and Simpson dominance showed a decreasing trend, while the Shannon-Wiener diversity and Pielou's evenness showed a gradual upward trend. The community similarity coefficient (C_J, C_S) showed a gradual increasing trend, with the maximum values in marsh meadows and marshes, and the minimum values in typical meadow and marsh meadow. The change in Cody index pattern showed a gradual trend-the total number of species in communities was gradually reduced and the species turnover rate decreased. Studies suggest that plant species composition, Pielou's evenness index, Simpson's dominance index, and Shannon-Wiener diversity index of Calamagrostis angustifolia wetlands in Sanjiang Plain are closely related to changes in water dynamics. β diversity is closely related to the pattern of water gradient, whereas the change in species richness is not correlated with water gradient. This may be associated with the biological characteristics of species and other environmental factors, and warrants further studies.