<p>Lower-frequency characteristics of the terahertz regime confer advantageous low photon energy for biochemical sensing while imposing inherent sensitivity constraints. Here, we demonstrate a terahertz asynchronous twin-comb sensor and an extra-spectrum sensing mechanism through cascading microchannel architecture within a metallic waveguide. The extra-spectrum sensing prefigures an enhanced sensitivity of 4 orders of magnitude compared to existing terahertz biosensing and surpasses its counterpart in the optical band. Hypersensitivities of 0.398 GHz mm² pg⁻¹ in trace detection manifest through the located characteristic resonance frequency beyond the spectrum domain. Additionally, we observed the photoisomerization of azo dye in the terahertz band with a photoresponse sensitivity of 0.91 GHz cm² mW⁻¹, opening possibilities for photoactive material-assisted terahertz sensors. In summary, we instantiate an asynchronous twin-comb sensing beyond the spectrum domain, offering a perspective for ultrasensitive sensing, and promising applications in optical frequency comb precision metrology, artificial intelligence photonics, and integrated sensing and communications.</p>