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Simple Summary: Biodiversity research on mountain ecosystems is mainly focused on vertebrates and vascular plants; therefore, we are still far from understanding the general diversity patterns of mountain insects. In this study, we surveyed and recorded butterflies in a typical subtropical mountain in the Chebaling National Nature Reserve, Guangdong Province, China, through grid-based monitoring across the entire region for two years, and then investigated their taxonomic and phylogenetic diversities. The results show that taxonomic diversity played a considerable role in mediating the effects of environmental changes on stability; stochastic processes are dominant in governing the composition of butterfly assemblages; increasing selective pressure mainly caused by abiotic factors enhanced the heterogeneous selection processes at higher elevations; placement trees could improve the accuracy compared to barcode trees in community phylogenetic analyses. Mountain ecosystems harbor evolutionarily unique and exceptionally rich biodiversity, particularly in insects. In this study, we characterized the diversity, community stability, and assembly mechanisms of butterflies on a subtropical mountain in the Chebaling National Nature Reserve, Guangdong Province, China, using grid-based monitoring across the entire region for two years. The results showed that species richness, abundance, and Faith's phylogenetic diversity decreased with increasing elevation; taxonomic diversity played a considerable role in mediating the effects of environmental changes on stability. Moreover, our results showed that stochastic processes are dominant in governing the assembly of butterfly communities across all elevational gradients, with habitats at an elevation of 416–580 m subjected to the strongest stochastic processes, whereas heterogeneous selection processes displayed stronger effects on the assembly of butterfly communities at 744–908 m, 580–744 m, and 908–1072 m, with abiotic factors inferred as the main driving forces. In addition, significant differences were detected between the barcode tree and the placement tree for the calculated β-NTI values at 416–580 m. Overall, this study provides new insights into the effects of environmental change on the stability and assembly of butterflies in Chebaling, which will be beneficial for biodiversity conservation and policy development. [ABSTRACT FROM AUTHOR]