ABSTRACTAs the climax community of karst in southwest China without human disturbance, the mechanisms through which biodiversity maintains ecosystem multifunctionality (EMF) in evergreen and deciduous broad‐leaved mixed forest are still unclear. This study employed structural equation modeling to analyze the correlations between EMF and three key variables: plant diversity, soil microbial diversity, and rock exposure ratio within this karst forest. Plant and soil microbial diversity were the main biotic drivers of EMF. Within these biotic factors, soil microbial diversity provided a preferable explanation for EMF compared to plant diversity, with fungal diversity and the co‐occurrence network complexity playing dominant roles. Functional, species, and phylogenetic diversity indices of plant explain the changes in EMF better than the community‐weighted mean of traits, indicating that the niche complementarity hypothesis holds greater relevance in explaining the biodiversity‐maintenance mechanism of EMF. Moreover, the rock exposure ratio directly affects EMF as well as indirectly through plant diversity. Collectively, soil microbial diversity, plant diversity, and rock exposure ratio account for 87% of the variability in EMF. Specifically, plant diversity, fungal diversity, and its co‐occurrence network complexity exerted a positive and direct impact on EMF, while bacterial diversity mainly affected EMF by positively contributing to fungal co‐occurrence network complexity. These findings emphasized the significance of comprehensive protection of aboveground and underground biodiversity and have strengthened our comprehension of the biodiversity‐maintenance mechanisms of EMF in karst forests.