We　proposed　a　terahertz　(THz)　surface　plasmon　resonance　(SPR)　sensor　with　Kretschmann　coupling　configuration　with　buffer　layer　covered　substrate　containing　doped　monolayer　graphene　(MLG)　directly　deposited　onto　the　coupling　prism.　The　performance　of　the　SPR　sensor　in　terms　of　detection　accuracy,　sensitivity　and　figure　of　merit　(FOM)　were　investigated　by　applying　angular　interrogation　technique.　The　results　demonstrated　that　the　prism/substrate/graphene　coupling　structure　has　higher　detection　accuracy　after　coating　a　low-permittivity　buffer　layer　underneath　graphene.　Moreover,　the　sensitivity　of　sensor　could　be　enhanced　using　larger　refractive　index　substrate.　Furthermore,　this　proposed　structure　could　effectively　overcome　the　challenge　that　sample　preparation　with　a　small　thickness　needs　to　be　maintained　in　Otto　geometry　based　sensor.　The　results　showed　the　maximum　sensitivity　of　the　THz　plasmonic　sensor　up　to　49.5　deg/RIU　along　with　a　FOM　of　8.76　RIU-1.　The　prominent　characteristics　of　the　proposed　structure　would　pave　the　way　for　high-performance　THz　plasmonic　sensors.