摘要： Terahertz (THz) perfect absorber with ultra-narrow bandwidth at near-unity absorbance has been a challenging issue due to mutual restriction of elemental requirements in design. Here, we propose an extremely sharp dual-band THz perfect absorber with ultrahigh Q-factor and near-unity absorbance by incorporating a Fabry-Perot micro-cavity into dielectric grating slit waveguide. Full-wave simulation demonstrates that the proposed structure not only achieves ultra-narrow band perfect absorption with an ultrahigh Q-factor of 4600 in THz regime, but also shows extraordinary sensing performance with a record figure of merit (FOM) of 2415 at THz frequencies. The optimized guided mode resonance leads to the ultrahigh Q-factor through suppressing the absorption and radiation losses. The enhanced interaction between the cavity localized fields and the analytes significantly improves THz sensing capability. Our design approach shows a promising future in simple and inexpensive THz biochemical sensors.