We　reported　the　surface　plasmon　resonance　(SPR)　on　doped　monolayer　graphene　(MLG)　for　terahertz　(THz)　sensing　of　refractive　index　of　testing　samples　using　a　prism-coupling　attenuated　total　reflection　configuration.　The　theoretical　detection　range　and　sensitivity　of　the　THz　plasmonic　sensor　were　investigated.　The　sensor　performance　in　terms　of　variation　of　detection　accuracy　with　different　refractive　index　was　explored.　It　was　demonstrated　that　the　Fermi　level　energy　of　MLG　and　also　the　gap　distance　between　the　prism　base　and　the　MLG　had　great　effects　on　the　performance　of　THz　sensing.　The　gap　distance　not　only　caused　the　coupling　resonance　frequency　shift　in　the　reflection　spectrum,　but　also　affected　the　full　width　at　half　maxima　(FWHM)　of　the　SPR　curve.　The　incident　angle　of　THz　radiation　was　also　the　key　impact　factor　of　the　detection　accuracy　of　the　sensor.　The　effective　SPR　frequency　could　be　tuned　by　controlling　the　incident　angle　or　the　Fermi　level　energy.　The　results　revealed　the　maximum　sensitivity　of　the　THz　plasmonic　sensor　up　to　6.65THzRIU(-1)　with　a　figure　of　merit　(FOM)　of　1187RIU(-1)　for　Fermi　level　energy　ranges　from　0.4　to　1.2eV,　making　the　sensor　potential　for　ultra-sensitive　SPR　sensing　in　the　terahertz　regime.