Aboveground biomass (AGB) is critical for ecological conservation and pastoral development in arid regions. Unraveling its formation mechanisms is foundational for sustainable grassland utilization in these areas, yet regional-scale investigations on this topic remain limited. Here, based on 216 grassland quadrats in the Aksu region of Xinjiang, we measured AGB, species diversity, functional traits, and environmental variables to disentangle the biotic and abiotic controls of AGB. Integrating regression analysis, hierarchical partitioning, and structural equation modeling (SEM), we elucidated the regulatory mechanisms of grassland AGB in arid regions by considering both biotic and abiotic factors. The results showed that: (1) AGB was positively correlated with species richness, Shannon-Wiener diversity, and Simpson diversity (P<0.001), but not with evenness, consistent with the niche complementarity hypothesis.(2) AGB exhibited significant positive linear relationships with community leaf carbon, nitrogen, and phosphorus contents (P<0.001), which is consistent with the prediction of the mass ratio hypothesis. (3) Among abiotic factors, AGB was significantly positively correlated with latitude, altitude, and annual precipitation (P<0.001). Conversely, it exhibited a significant negative correlation with mean annual temperature (P<0.001). Notably, among all studied biotic and abiotic factors, altitude exerted the most significant influence on AGB, accounting for 18.21% of the observed biomass variation. (4) Geographical factors indirectly but significantly influenced AGB via climatic factors and community functional traits, while climatic factors and community functional traits had significant direct effects on AGB. This revealed a hierarchical AGB regulatory mechanism: "geographical pattern dominance-climatic stress intensification-community functional trait response". These findings advance understanding of the multidimensional controls of AGB and provide a framework for sustainable grassland management in arid mountain-basin systems.