Abstract: An annular wetland typical in the Sanjiang Plain was chosen as a case study on wetland biodiversity.Based on data of the previous field investigations and studies,status quo of the habitat diversity and species diversity of annular wetlands and causes of their formation were studied and the relationship between the two were explored.The results show that the habitat diversity was rather complicated and rich,and was formed and maintained through interaction between hydrography and topography on various spatial and temporal scales.The richness index and Shannon-Wiener index increased at first and then decreased along the radius from the center to the rim of the annular wetland,reaching the apex with the Carex casiocaepa community and getting lower with the Carex pseudocuraica and Salixbrachypoda-Salixmyrtilloides communities.The species diversity was rich in the annular wetland.Large-sized soil animals were the highest in diversity and homogeneity indexes and the lowest in superiority index in the reed-sedge communities,and so were medium-and-pint-sized soil animals in the Calamagrosticsangustifolia paludified meadow.The species diversity is closely related to the habitat diversity in the annular wetland of the Sanjiang Plain.

Key words: habitat diversity, species diversity, annular wetland, Sanjiang Plain