The rapid expansion of grey space (GS) profoundly impacts the distribution and sustainability of urban green space (UGS), especially in water-scarce arid and semi-arid regions. However, the relationship between GS and UGS and its variation across cities with different socio-political and environmental contexts remains poorly understood. This study examines eight Asian arid and semi-arid cities categorized by distinct characteristics: Group1 (Lanzhou, Xi'an; geographically-constrained), Group2 (Baghdad, Tehran; conflict-affected), Group3 (Tashkent, Dushanbe; stable expanding), and Group4 (Beirut, Damascus; small-scale). We analyzed the spatiotemporal dynamics of GS and UGS from 1985 to 2022 using urban expansion intensity, net changes from UGS to GS, etc. We then employed the Geodetector and landscape indices to evaluate the influence of GS spatial pattern on UGS. The results show that: (1) GS expansion intensity depends on land availability, which is constrained by topography, climate and social stability; (2) Socio-political stability fundamentally shapes urban expansion patterns, leading to divergent trajectories from security-driven dispersal to policy-induced optimization; (3) The impact mechanisms of GS on UGS exhibit significant scale dependency and regional heterogeneity: terrain dominates at regional scale, while GS configuration, socio-economic factors, and their interactions play varied roles across cities; (4) Context-specific planning strategies are proposed: rigid regulation and internal optimization for geographically-constrained cities, multi-centric decentralization for conflict cities, refined morphological management for stable expanding cities, and three-dimensional greening for land-scarce cities. Our findings deepen the understanding of GS-UGS dynamics and offer valuable insights for optimizing spatial planning and promoting sustainable land use in arid and semi-arid urban environments.