It mainly introduces the function of the dual-band back-adding film of type-II superlattice detector chip in the detector and its membrane system design and preparation process. The function of the film is to reduce the reflectivity of infrared light on the surface of the detector chip in the response band (3--5 m and 7--9 m), enhance the light response rate of the chip, that is, improve the performance of the chip. The main technical problems in the design and preparation of the back-adding film are studied and solved. According to the use environment of the detector, the corresponding firmness experiment, test and analysis of the film are carried out to draw conclusions. The results show that the spectral responsivity and firmness of this film can fully meet the requirements of the detector. At present, the preparation process of the back-adding film is an indispensable process step in the production process of type-II superlattice detector. The application prospect is good.