We　report　a　high-resistivity　silicon　(HR-Si)　prism　coupled　terahertz　(THz)　spoof　surface　plasmon　polaritons　(SSPPs)　on　flat　subwavelength　metasurface.　Using　a　high　refractive　index　prism　as　an　external　coupler,　a　more　tightly　confined　SSPPs　mode　can　be　excited　in　a　smaller　resonant　cavity,　leading　to　strong　light-matter　interaction.　Besides,　theoretical　analysis　and　experimental　results　have　both　indicated　that　the　SSPPs　resonance　response　to　the　filling　patterns　of　analyte　in　the　resonant　cavity　are　quite　different.　In　particular,　we　have　found　that　the　interaction　between　analyte　and　SSPPs　wave　can　be　maximized　when　the　analyte　filled　with　the　whole　resonant　cavity　and　a　higher　sensitivity　for　THz　sensing　can　be　obtained.　A　high　sensitivity　varied　from　0.31　THz/RIU　to　0.85　THz/RIU　is　predicted.　Furthermore,　these　SSPPs　modes　exhibit　high　Q-factor,　and　characteristic　spectra　of　water　caused　by　surface　plasmon　resonance　(SPR)　are　observed,　which　is　significant　in　promoting　the　THz-SPR　sensing　of　polar　liquids　or　aqueous　analytes　with　THz　metasurfaces.　©　2019　Optical　Society　of　America