Graphene　plasmons　exhibit　much　larger　confinement　and　relatively　long　propagation　distances　at　terahertz　(THz)　regime,　with　the　advantage　of　being　highly　tunable　via　electrical　and　chemical　doping.　Thus,　it　provide　a　suitable　alternative　to　noble-metal　plasmons.　We　investigated　the　Otto　configuration　based　THz-plasmonics　sensor　using　a　continuous　suspended　monolayer　graphene　(MLG)　and　graphene/insulator　stacks　(GIS)　by　finite　element　method　(FEM).　We　revealed　that　a　continuous　suspended　MLG　based　sensor　has　extremely　high　detection　accuracy　and　promising　prospect　of　application　in　ultrasensitive　THz-plasmonics　gas　sensing.　In　addition,　more　layers　of　graphene　will　result　in　higher　detection　accuracy,　but　lower　sensitivity　and　FOM　of　the　sensor　using　GIS.　©　2017　IEEE.