The　Bukit　Timah　granite　(BTG),　which　occupies　one　third　of　Singapore　land　area,　has　excellent　engineering　properties　and　is　suitable　for　the　development　of　underground　space　for　various　purposes.　Microwave-assisted　rock　breakage　has　been　proven　to　be　a　promising　method　for　solving　the　problems　of　low　operating　efficiency　and　excessive　tool　wear　during　mechanical　excavation　in　hard　and　highly　abrasive　rock　masses.　In　this　paper,　the　weakening　effect　of　microwave　irradiation　on　BTG　and　its　impact　on　mechanical　excavation　performance　were　studied　by　laboratory　tests　and　empirical　models.　The　results　showed　that　the　longer　the　microwave　irradiation　time,　the　greater　the　sample　mass　loss　and　volume　change.　Corresponding　to　the　variation　of　crack　characteristic　and　sample　volume,　the　ultrasonic　velocity　and　UCS　of　BTG　decrease　with　the　irradiation　time.　The　mass　change　can　be　explained　by　the　escape　of　contained　water　at　high　temperature,　while　the　weakening　mechanism　is　mainly　related　to　the　differential　expansion　among　biotite,　quartz　and　other　minerals.　Meanwhile,　microwave　irradiation　can　also　decrease　the　CAI　value　of　BTG　to　some　extent　due　to　the　weakened　connection　between　mineral　particles　and　micro　cracks　generated　in　the　sample.　Prediction　on　the　mechanical　excavation　performance　of　roadheader　shows　that　the　excavation　rate　and　tool　consumption　can　be　greatly　improved　by　microwave　irradiation.