Temperature　disturbance　could　cause　deterioration　of　rock　mechanical　parameters,　and　it　is　of　great　significance　to　establish　the　corresponding　quantitative　characterization　method　of　rock　damage　to　reveal　the　damage　and　instability　mechanism　of　rock　and　guide　engineering　practice.　In　this　study,　fine-grained　granite　was　selected　as　the　object　of　research　and　has　undergone　different　thermal　treatments　with　different　high-temperature　and　cooling　methods.　The　formation　and　development　of　pores　and　microcracks　were　observed　and　analyzed　by　field　emission-scanning　electron　microscopy　(FE-SEM)　and　low-field　NMR　scanning　experiments.　Then,　the　rock　damage　coefficients　based　on　macro　and　microdamage　mechanisms　were　calculated,　and　the　mechanism　of　microdamage　was　specifically　analyzed　based　on　the　change　of　the　characteristics　of　the　small,　medium,　and　large　pores　of　the　rock　and　the　characteristics　of　fractal　dimension.　The　results　showed　that　the　porosity　of　granite　increased　slowly　first　and　then　rapidly　with　the　increase　of　heat　treatment　temperature,　and　the　proportion　of　medium　and　large　pores　made　the　most　significant　contribution　to　the　improvement　of　porosity.　The　damage　value　of　granite　increased　continuously　with　the　heat　treatment　temperature.　The　damage　value　of　liquid　nitrogen　(LN2)　cooling　was　the　highest,　followed　by　water　and　natural　cooling.　After　heat　treatment　and　cooling　treatment,　the　pore　distribution　of　granite　had　good　statistical　fractal　characteristics,　and　the　fractal　dimension　decreased　with　the　increase　in　heat　treatment　temperature,　and　the　fractal　dimension　of　the　rock　cooled　by　liquid　nitrogen　was　the　highest,　followed　by　water　cooling　and　natural　cooling.