Karst tiankengs are not only a rare geological wonder in the world with their huge negative-landform in the earth's surface, but an ideal region to explore plant species diversity and spatial distribution, which attribute to an unique habitat formed by their trapped topography. This study analyzed the grassland plant community in the "Bajiaxiantang" karst tiankeng, which is a moderately degraded habitat in the Zhanyi tiankeng group in Yunnan Province. The alpha diversity index was applied to analyze the spatial distribution characteristics of the grassland plant community species in the bottom of this habitat, and the similarity of the grassland plant community was compared to that of the degraded doline near "Bajiaxiantang" and the outside area of the "Damaosi" primitive tiankeng using a beta similarity coefficient. The results showed that: (1) in the bottom of the moderately degraded karst tiankeng, the species richness and diversity of the grassland plant community presented an "inverse depression effect", where the species richness and diversity were higher in the center and lower at the edge; (2) compared with soil resources, the growth and spatial distribution of plant communities in the karst tiankeng were possibly more sensitive to light resources, and the spatial heterogeneity of light caused by the height difference of the tiankeng pit wall resulted in a "circle effect" of the grassland plant species diversity; (3) the isolated and protective function on plant communities by the unique trapped topography of karst tiankengs even be reflected in degraded tiankengs, and are more protective for ombrophyte. With global climate change, karst tiankengs should be served as important regional small-scale ecological sanctuaries to protect these unique ecosystems, even though they are degraded.