During　the　development　process　of　deep　geothermal　energy　(Hot　dry　rock,　HDR),　the　high-temperature　granite　matrix　in　the　wellbore　needs　to　withstand　thermal-cooling　cycles,　easily　leading　to　wellbore　instability　and　collapse.　Considering　the　actual　drilling　engineering　using　different　types　of　drilling　fluid,　thermal-cooling　cycle　experiments　(cycles　of　1,　2,　4,　8,　12　times)　of　granite　were　conducted　using　natural,　water,　and　liquid　nitrogen　(LN2)　cooling　methods.　Based　on　the　low-field　nuclear　magnetic　resonance　measurement,　the　development　and　evolution　of　pore　size,　quantity,　and　distribution　under　different　cooling　ways　and　thermal　cycles　were　studied,　and　the　changes　in　rock　mechanical　parameters　under　different　cooling　methods　and　thermal　cycling　conditions　were　quantified.　Then,　the　rock　damage　coefficients　based　on　the　variation　patterns　of　different　types　of　pores　were　calculated,　and　a　quantitative　relationship　was　established　among　T2　spectrum,　pore,　and　mechanical　damage.　Finally,　the　proton　weighting　method　was　used　to　analyze　the　changes　in　the　internal　bearing　area　of　rocks,　revealing　the　deterioration　mechanism　of　rock　mechanical　properties　under　different　cooling　ways　and　thermal　cycling　conditions.　The　results　showed　that　the　porosity　increased　with　thermal　cycles　during　the　thermal-cooling　cycle　treatment,　and　the　growth　rate　was　first　fast　and　then　slow.　Under　the　same　number　of　thermal　cycles,　the　groundwater　cooling　porosity　was　the　highest,　followed　by　LN2　and　natural　cooling.　The　relationship　between　T2　spectral　area　and　rock　damage　was　established　on　this　basis,　and　the　rock　damage　value　increased　with　thermal　cycles　in　an　exponential　function,　and　the　proportion　of　large　pores　significantly　affected　the　pore　damage.　A　relationship　(positively　proportional　linear)　between　pore　damage　and　the　mechanical　degradation　coefficient　of　high-temperature　cooling　cycle　granite　and　a　method　for　predicting　rock　degradation　through　pore　damage　were　established,　the　deterioration　mechanism　of　rock　mechanical　properties　under　thermal　cycle　treatment　was　revealed.