Karst landforms in Yunnan is one of the priorities of karst desertification management in Yunnan-Guangxi-Guizhou provinces. Exploring the community assembly characteristics of succession stages under climate warming and humidification is essential for ecological restoration and sustainable forest management. In the context of climate warming and humidification, karst tiankeng was taken as the natural "open top chamber" and the underground forest on tiankeng's south slope as the community succession apex. The community assembly mechanism in successional sequences (mixed coniferous forest outside the tiankeng, evergreen broadleaf forest outside the tiankeng, and evergreen broadleaf forest inside the tiankeng) is explored, which is based on phylogenetic and functional traits. The results showed that: (1) the phylogenetic signals of plant traits were lower in the early succession stage, with significant phylogenetic signals for leaf aspect ratio, leaf area, leaf thickness, and specific leaf area, while functional traits were less conserved. In the late succession stages, the plant traits evolved convergently and none of the phylogenetic signals were significant. (2) The phylogenetic diversity of plant communities gradually decreased during the succession stage, and was significantly and positively correlated with species richness. (3) The phylogenetic indices for NRI (Net Relatedness Index) and NTI (Nearest Taxon Index) shifted from less than 0 to greater than 0, and the functional trait structure indices for TraitSESMPD (Standard Effect Size of Mean Pairwise Trait Distance) and TraitSESMNTD (Standard Effect Size of Mean Nearest Trait Distance) were both greater than 0. Community assembly processes in successional stages were driven by environmental filtering and interspecific interaction. (4) Phylogenetic and functional trait structure indices were negatively correlated with soil water content, total phosphorus, and available potassium. Under the successional stage, the community tended to assemble when soil water and nutrients were limited, and the community assembly process supported the ecological niche hypothesis. Our findings will contribute new perspectives to the community assembly mechanism of karst plant and provide scientific reference for community structure optimization.