The underground forest of the degraded karst tiankeng is mainly distributed on the inverted stone slopes of different slope aspects. In this paper, we explore the role of environmental filtering, competitive exclusion and random processes in the underground forest community assembly in karst tiankeng, which is helpful to reveal the value of species refuge of the karst tiankeng, and provide scientific reference for the protection of tiankeng plant diversity and ecological restoration of surface outside the karst tiankeng. This study took the Shenxiantang degraded tiankeng in Zhanyi, Yunnan as an example. Mainly combined with functional trait ecology and phylogenetic methods, we investigated the soil environmental characteristics, functional diversity variation, phylogenetic signals and phylogenetic diversity of plant communities in different slope aspects. The results showed that:(1) the soil environment varied greatly from slope to slope, with the greatest degree of variability on the eastern slope compared to the southern and northern slopes. (2) The dominant populations varied considerably across the slopes, among them, Fagaceae plants had relatively large abundance, Quercus pannosa had the highest population abundance on the northern and eastern slopes, and Cyclobalanopsis glauca had the highest population abundance on the southern slope. The functional traits of dominant species were different across the slope aspects, the degree of variation was medium on the whole, but the degree of variation was the largest for the dominant species on the southern slope. (3) Leaf width, leaf aspect ratio, leaf thickness and specific leaf area had strong and significant phylogenetic signals (K>1, P<0.05), which showed that the evolution of traits was strongly influenced by genetic factors; While leaf length, leaf area, leaf dry weight, leaf dry matter content and maximum tree height had weak phylogenetic signals (K<1), which indicated that the evolution of traits was greatly influenced by habitat factors. (4) The assembly process of plant community in each slope acpect was dominated by the environmental filtering and competitive exclusion effects of niche theory. During the transition from the slope aspects from north to east and south, the environmental filtering effect of plant community assembly gradually decreased, and the competitive exclusion effect gradually increased. The northern slope may be dominated by xerophytes as the mainly constructive species for a long time, while the southern slope may eventually succeed to a subtropical humid climatic top community in the future.