The north-south oriented Helan Mountain Range in northwestern China forms a pronounced climatic divide, with a relatively mesic desert steppe on its eastern slope and a hyper-arid desert on its western slope. How this sharp east-west contrast interacts with elevation to shape soil arthropod biodiversity remains poorly understood. Here, we examined elevational patterns of ground-dwelling arthropod alpha and beta diversity across six elevation bands (1400-2650 m) on both slopes. We reveal a clear scale-dependent decoupling between diversity components. Beta diversity increased consistently toward the summit on both slopes, indicating robust, elevation-driven species turnover largely insensitive to slope aspect. In contrast, alpha diversity showed strongly divergent patterns: a bimodal distribution on the eastern slope and a monotonic decline on the western slope, indicating that slope aspect acts as a key local filter for community assembly. Mechanistically, diversity on the eastern slope was jointly regulated by plant height and elevation-mediated changes in soil phosphorus and moisture, highlighting biotic-abiotic linkages. On the western slope, beta diversity was primarily driven by total soil nutrients and salinity, reflecting direct abiotic filtering under arid stress. Structural equation models confirmed that elevation influences arthropod diversity through distinct, slope-specific indirect pathways. By disentangling slope-dependent local assembly processes from slope-independent elevational turnover, this study advances understanding of how topography modulates biodiversity across spatial scales in arid mountains. Our findings underscore the necessity of slope-specific conservation strategies in north-south oriented mountain systems facing increasing climatic aridity and variability.