Based on field investigation and by using Levins formulae, the niche breadth and niche overlap of dominant plant populations in Sanjiang Plain marsh wetland were studied. The results showed that the niche breadth of test marsh wetland plants decreased in the order of Carex pseudocuraica (0.907) > C. lasiocarpa (0.864) > Glyceria spiculosa (0.855) > Deyeuxia angustifolia (0.500) = Menyanthes trifotiata (0.500) = Naumburgia thyrsiflora (0.500) = Anemone dichotoma (0.500). The niche overlaps among C. lasiocarpa, C. pseudocuraica, M. trifotiata and N. thyrsiflora, and among G. spiculosa, D. angustifolia and A. dichotom were comparatively large, indicating an intense competition among them, while the niche overlap between M. trifotiata and N. thyrsiflora, and D. angustifolia and A. dichotoma was 0, suggesting their different manner of environmental adaptation. Soil water was the dominating environmental factor of wetland plant ecological adaptability. The niche fitness of plant to environment depended on the combined effect of soil water and nutrients. The niche characteristics of marsh wetland plants could better indicate their ecological adaptability and distribution range.