Geothermal reinjection is essential for sustainable geothermal energy utilization, yet reservoir scaling triggered by water–rock interactions remains a critical obstacle to long-term operation, and the coupling mechanisms between scaling and pore-permeability evolution remain unclear. In this work, high-temperature and high-pressure reinjection experiments were conducted on natural and artificial sandstone cores, combined with ICP-OES, XRD, SEM and X-CT characterizations, to reveal scaling mechanisms and porosity-permeability evolution patterns. Temperature-ramp tests with deionized water (50–100 °C) demonstrate that thermal expansion and thermally induced clay mobilization reduce permeability by 62.52% in natural cores and 68.76% in artificial cores, verifying that temperature elevation alone can induce significant permeability impairment. Under 80 °C synthetic formation water reinjection, water–rock interaction-driven scaling decreases permeability by over 15% (natural cores) and 29.34% (artificial cores). Calcite dominates scaling products, showing dispersed distribution in natural cores and preferential pore-throat deposition in artificial cores. These findings clarify the coupling law of mineral composition and scaling deposition on pore structure evolution, and can provide a scientific reference for scaling prevention and field reinjection management of sandstone geothermal reservoirs.