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.