<p>Chinese metropolitan areas face two intertwined challenges: the concentration of high-quality healthcare resources and administrative frictions that constrain cross-jurisdictional care. Using the Xi’an metropolitan area as a case, this study examines how administrative boundary frictions shape multi-tier healthcare accessibility and equity through a scenario-based Gaussian-enhanced two-step floating catchment area (G2SFCA) approach. We compare a boundary-constrained scenario with a boundary-relaxed scenario to estimate the potential redistribution of healthcare opportunities under metropolitan integration. The results show that boundary relaxation can weaken the core–periphery pattern of access, but the gains are spatially uneven and tier-dependent. Equity improvements are strongest for tertiary care and are concentrated in peripheral subdistricts with relatively low travel-time impedance, whereas some core and inner-suburban areas experience reduced per-capita accessibility as metropolitan catchments expand. An age-related vertical equity extension further shows that subdistricts with higher shares of older adults tend to have lower healthcare accessibility under both scenarios, and that this negative relationship becomes stronger under boundary relaxation. The findings suggest that boundary relaxation may reallocate service pressure rather than universally improve access, and that improvements in horizontal equity do not necessarily translate into gains in age-related vertical equity. The proposed framework helps identify underserved and ageing-related equity-vulnerable subdistricts and provides evidence for cross-jurisdictional healthcare planning and metropolitan governance.</p>