This paper introduces an ultra-wideband terahertz thermal absorber, composed of double-layer square groove doped silicon cubes, exhibiting polarization independence. Within the frequency range of 0.9–9.7 THz, the absorption efficiency exceeds 95 %, and within an incident angle range of 0–67°, the absorption rate still maintains above 90 % in the frequency range of 1.4–9.7 THz. Through finite element method (FEM), this study clearly demonstrates the distribution of electromagnetic energy within the thermal absorber and thoroughly elucidates its notable absorption efficiency via impedance matching theory. Compared to existing technologies, the proposed double-layer square groove thermal absorber shows superior performance in terms of absorption efficiency and bandwidth. This ultra-broadband terahertz thermal absorber is highly applicable in areas including biomedical imaging, thermal imaging, and wireless communication. Considering its excellent manufacturing tolerance, this thermal absorber is significant for application in real industrial production.