Spatial Linkage Networks and Spatial–Functional Urban Integration for Sustainable Metropolitan Development: Evidence from the Beijing Metropolitan Area

Sustainable metropolitan development depends on the effective transformation of spatial linkages into functional integration, including efficient commuting, equitable public service accessibility, spatial connectivity, and cross-boundary governance. However, existing studies often rely on administrative boundaries, static indicators, or coarse spatial units, limiting their ability to identify fine-scale spatial heterogeneity and explain how metropolitan linkages shape functional integration. Taking the Beijing Metropolitan Area as a case study, this study develops an integrated framework combining metropolitan boundary delineation, spatial linkage network analysis, and spatial–functional urban integration assessment. SDGSAT-1 nighttime light data (NTL) and transport accessibility analysis are used to identify Beijing’s main urban center and delineate the metropolitan extent. Population flow, resource linkage, and comprehensive spatial linkage networks are constructed to characterize network structure and nodal hierarchy. Spatial–functional urban integration is then evaluated at the township/subdistrict scale from three dimensions: commuting integration, public service accessibility, and spatial connectivity. The results show that Beijing’s main urban center extends beyond the traditional central urban area, and the metropolitan area exhibits concentric attenuation, directional differentiation, and heterogeneous expansion. Although cross-boundary linkages have emerged, the network remains dominated by Beijing’s core functions, with localized support from Tianjin and absorptive roles of adjacent Hebei areas. Spatial–functional integration advances primarily along major linkage corridors and key nodes, forming a hierarchical pattern consistent with the comprehensive spatial linkage network. These findings provide fine-scale spatial evidence for optimizing functional corridors, strengthening secondary nodes, improving public service allocation, and promoting cross-boundary spatial governance in sustainable metropolitan development.