We　experimentally　demonstrated　a　corrugated　metallic　metasurface　based　tunable　perfect　absorber　for　terahertz　(THz)　frequencies　in　a　total　internal　reflection　geometry.　The　absorbance　is　strongly　depend　on　the　central　layer　of　this　three-layer　absorber,　which　provides　a　feasible　approach　to　tune　the　absorption.　In　particular,　there　exist　an　optimal　gap　that　enables　a　perfect　absorption　at　specific　frequency.　Due　to　the　simple　1D　geometric　structure　of　metasurface,　its　absorption　frequency　can　be　easily　tailored　over　a　wide　frequency　range　(0.625-1.499　THz).　More　importantly,　the　modulation　of　the　effective　refractive　index　and　loss　of　medium　environment　can　be　accepted　as　an　alternative　approach　for　the　absorption　properties　modulation.　This　prism　coupling　absorber　provides　a　new　route　for　modulation　of　the　absorption　characteristics　with　potential　applications　in　biological　sensing.　(C)　2019　Optical　Society　of　America　under　the　terms　of　the　OSA　Open　Access　Publishing　Agreement