Soil organisms, with their extremely high diversity, play essential roles in maintaining soil health and supporting key ecological processes. The importance of soil organisms in the aridity system is widely recognized for sustaining soil multifunctionality, yet it remains poorly understood how this relationship is affected by increasing aridity in dryland ecosystems. Here, we combined a large-scale field survey across 132 sites spanning a 4,500 km aridity gradient in northern China, from dry-subhumid to arid regions (aridity index, 1-AI: 0.33-0.95) with a controlled microcosm experiment to investigate how aridity influences the biodiversity-multifunctionality relationships. We quantified the diversity of major soil organism groups-bacteria, fungi (including total, saprotrophic, and symbiotrophic fungi), protists and invertebrates-as well as overall multidiversity, and measured key soil functions related to carbon, nitrogen, and phosphorus cycling. We found that both soil biodiversity and soil functioning declined significantly with increasing aridity. Despite these declines, positive relationships between soil biodiversity and multifunctionality were consistently observed and became significantly stronger under higher aridity. Notably, as aridity intensified, spatial heterogeneity in soil environmental conditions and in soil community composition increased. This heterogeneity was strongly and positively associated with the strength of biodiversity-multifunctionality relationships across all soil organism groups. These findings reveal a key mechanism by which aridity reinforces biodiversity-function coupling through enhanced environmental and biotic heterogeneity. Overall, our results highlight the importance of conserving not only belowground biodiversity but also the spatial heterogeneity of soil communities and habitat conditions to sustain ecosystem multifunctionality in drylands under ongoing climate-driven aridification.