Aiming at the design and application of the interband cascade structure in long wavelength detection, E-k relation and energy band information of multi-quantum-well relaxation region were calculated by using two-band model under the envelope-function approach and transfer matrix method. The structure of relaxation region was specially optimized. On the premise of the tunneling process of photon-generated carriers in relaxation region, the electric field of absorption region was reduced to suppress generation-recombination current and tunneling current, and to improve the electrical performance of device. The quantum efficiency of prepared double-stage interband cascade photodetector was up to 20% at 10.5 μm, which proved that the relaxation region and tunneling region had well transport of photoexcited carriers. The 50% cutoff wavelength of the device was 11.5 μm at 80 K, which was the longest wavelength infrared detector of interband cascade structure at 80 K.