Forests can provide multiple ecosystem functions simultaneously that are related to biomass production, soil carbon stock, and nutrient cycling, and plant diversity is often important for the maintenance of ecosystem multifunctionality (EMF). However, the underlying mechanisms involved in regulating relationships between species richness and abundance and EMF in a subtropical coniferous forest still remain unclear. Using a randomization test and structural equation model, we show that multiple plant diversity attributes, stand structural complexity, dominant species, and soil conditions affect EMF from 94 plots in Pinus kesiya dominant stands in southwest China. Our results indicate that the direct effects of species richness on EMF were stronger than the effects of woody individual abundance, and species richness increased with increasing EMF. By contrast, EMF declined with increasing total woody species abundance, while functional diversity and dominant species abundance did not exhibit significant effects on EMF. However, Castanopsis echidnocarpa abundance indirectly increased EMF via species richness and total woody species abundance. Simultaneously, our findings suggest that stand structural complexity mediated the relationship between plant diversity and EMF, and species richness, total woody species abundance, and EMF increased strongly with increasing stand structural complexity. Higher soil pH and soil water content were associated with greater stand structural complexity and EMF, which were important drivers in explaining variation in EMF. Our results show that higher species richness promoted EMF provisioning, and provide critical insights to predicting the possible consequences of EMF with a change in biotic and abiotic factors.