The nested subset pattern (nestedness) has been widely used to explain species distributions in island and fragmented systems. Mountain sky islands serve as critical natural laboratories for understanding the evolutionary consequences of geographic isolation and climate change, but their species distribution patterns remain poorly understood. Compared to lowland fragmented habitat islands, biodiversity patterns in mountainous regions are more complex, and traditional drivers may not adequately explain the nestedness of sky islands. To uncover the underlying mechanisms of mountain biodiversity, this study sampled amphibians across 30 sky islands in the Dabie Mountains, China, aiming to explore the nestedness of amphibian assemblages and their formative mechanisms from taxonomic, functional, and phylogenetic perspectives. Taxonomic nestedness was quantified by constructing species incidence matrices and calculating nestedness metrics, while functional and phylogenetic nestedness were assessed by integrating species similarities in ecological traits and phylogenetic relationships. Seven sky island characteristics and eight amphibian species traits were then selected as predictors of nestedness. Results showed that amphibian assemblages in the Dabie Mountains exhibited significant nestedness across taxonomic, functional, and phylogenetic dimensions. Spearman rank correlations between the selected predictors and nestedness ranks revealed that sky island area, climate stability, and species traits associated with extinction vulnerability were strongly correlated with taxonomic nestedness. Additionally, sky island area and climate stability significantly influenced functional and phylogenetic nestedness. These findings support the "selective extinction" and "habitat nestedness" hypotheses. To explain these results, we propose a climate stability hypothesis specific to sky islands: stable climatic conditions sustain greater species diversity, whereas climatic instability exacerbates the extinction of lineages lacking adaptive traits. This study presents the first empirical evidence of nestedness in sky island systems, extending classical hypotheses beyond lowland ecosystems, and offering new insights into climate-driven assembly mechanisms affecting vulnerable taxa. Our results indicate that conservation efforts should prioritize sky island regions with high climate stability, large areas and diverse habitats as core areas for biodiversity protection. Moreover, species with narrow elevation ranges, large area requirements, high habitat specificity, and long larval stage durations should also be preserved to prevent local extinction.