The negative topographic slopes in a degraded karst tiankeng play an important role in protecting plant diversity. Studying the functional diversity of soil microbial communities on slopes aids in clarifying the ecological characteristics and spatial distribution of soil, to explore the relationship between soil microorganisms and plant diversity, and to assess the biodiversity of degraded karst tiankengs. The degraded tiankeng "Bajiaxiantang" of Zhanyi county in Yunnan Province was selected as the study site. A Biolog MicroPlate was used to analyze the functional diversity of the soil microbial community in the slope environment and how soil microbial community functional diversity varied with gradient (pit, uphill, mid-slope, downhill, and pit bottom). The results showed that the functional diversity indices (Shannon, Simpson, and McIntosh) and the carbon source utilization degree of the soil microbial communities at the pit bottom were slightly higher than those at the pit level and were significantly higher than those at the mid-slope level, showing a trend of downhill > mid-slope > uphill. The main sources of carbon used by soil microbial community on slope are sugars and amino acids.. The two principal components account for 31.9% and 28.7% of all variance and can distinguish the carbon source utilization of soil microbial communities in different slope positions. The α diversity index of herbaceous plants inside and outside the pit was significantly different. The diversity index of woody plants on the slope was significantly different (P < 0.05), and the diversity index of the woody plants was highest downhill. The β diversity index showed that the habitats of the different slopes are different and that the habitats of the downhill area are the most diverse. The study found that the unique negative topography and soil environment of the degraded karst tiankeng talus slopes led to to higher plant diversity and higher functional diversity of soil microbial communities, and that the spatial distribution of the two is consistent. Thus, special attention should be paid to the value of underground forest resources on the talus slopes.