Abstract: We carried out niche monitoring and analysis of plant populations under the forest community in the talus slope ecotone of a typical moderate-degraded Bajiaxiantang tiankeng to provide scientific references for biodiversity conservation and vegetation restoration in degraded tiankeng area. The results showed that soil ammonium, available potassium, and available phosphorus signifi-cantly affected species distribution, which explained 37.4%, 32.8%, 29.3% of the total variation, respectively. With the change of talus slope of tiankeng (pit, uphill, mid-slope, downhill and pit bottom), life form of understory plants changed from evergreen and xerophytes to evergreen and hygro-mesophytes, with the niche overlap of herbs being larger than that of shrubs. Shrubs of Viburnum congestum and Campylotropis polyantha, and herbs of Arisaema erubescens and Arthraxon hispidus had wide ecological amplitude and strong resistance, which occupied the upper layer of the shrub and herb layers. Shrub Cornus oblonga and herb Geranium nepalense, Agrimonia pilosa lost the competitiveness with increasing soil alkalinity. Niche characteristics of understory dominant species in Bajiaxiantang were closely related to the canopy structure of mixed trees, ecological strategies of shrub and herb species, unique habitat of tiankeng, and the importance value of dominant species.