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It is shown that active-tunable terahertz absorbers can be realized in a sandwich-structured system comprising an ultrathin dielectric film (polyimide) on a temperature-sensitive substrate (InSb) with a metal film on the back by utilizing the intrinsic carrier density (N) variation in InSb.When increasing the temperature from 250 to 320 K,N in InSb varied from ～5.50×1015 to ～2.98×1016cm-3.Fixing the thickness of dielectric film with the value of 1.37 μm,the absorption peak shifted from 1.41 to 3.29 THz while keeping absorption higher than 99％.This active tunability can respond to even a slight temperature perturbation,and shows polarization insensitivity as well as high tolerance of incidence-angle (absorption peak can still exceed 90％ even the incidence angle reaches 60°).Besides,the refractive index of polyimide (PI) has thermal stability at the terahertz range and the merit of good workability.These characteristics guarantee the stability of active-tunable performance.The peculiarities and innovations of this proposal promise a wide range of high efficiency terahertz devices,such as thermal sensors,spatial light modulators (SLMs) and so on.