This　study　investigates　the　pore　structure　modification　of　coal　under　thermal　steam　circulation　injection　using　nuclear　magnetic　resonance　(NMR)　and　low-temperature　nitrogen　gas　adsorption　techniques.　Results　demonstrate　that　steam　stimulation　promotes　pore　network　development　by　increasing　T2　spectral　area　(indicative　of　pore　connectivity)　and　fractal　dimension　(reflecting　surface　roughness),　while　simultaneously　inducing　localized　thermal　stress-driven　pore　collapse,　evidenced　by　permeability　reduction　and　pore　volume　redistribution.　Analysis　of　injection　parameters　reveals　a　critical　trade-off:　extended　steam　exposure　enhances　macropore　dilation　(specific　surface　area　increased　by　∼30%),　yet　repetitive　cycles　accelerate　thermal　efficiency　decay　due　to　steam　condensation　hysteresis.　NMR-derived　T2　cutoff　values　and　mass　balance　data　further　quantify　dynamic　interactions—longer　injection　durations　amplify　mesopore　connectivity　(pore　throat　growth　rate　+18%　vs　baseline),　but　cyclic　operations　reduce　effective　wetting　range　by　15%-22%　per　additional　cycle.　Crucially,　optimal　steam　conformance　requires　balancing　injection　duration　and　frequency　to　mitigate　microscale　structural　damage　while　sustaining　macro-permeability　gains.　These　findings　highlight　thermal　steam＇s　dual　role　in　coal　seam　gas　extraction:　enhancing　methane　desorption　capacity　through　controlled　porosity　engineering,　yet　necessitating　real-time　parameter　adjustments　to　counterbalance　thermal　degradation　effects.　The　thermal　modification　technology　using　hot　steam　not　only　optimizes　resource　use　efficiency　to　the　fullest　extent　but　also　seamlessly　aligns　with　China＇s　strategic　goal　of　carbon　neutrality.　©　2025　Author(s).