In this paper, the development progress of multi-band infrared detector applications is introduced, with a focus on the readout circuit design tailored for infrared devices capable of simultaneous multi-band signal detection. The image input level is identified as one of the critical modules in the design of the readout circuit for multi-band infrared detectors. A Buffered Direct Injection (BDI) type pixel input level circuit structure is adopted, characterized by high stability and low equivalent input impedance, which effectively enhances the injection efficiency of the long-wave detector and provides a stable operating conditions for optoelectronic detection components. The full circuit design and simulation are completed using a CMOS 0.18 μm 5 V standard process. The results demonstrate that the circuit operates normally at a cryogenic temperature of 70 K, with a 40 μm pitch incorporating four-band pixel input stages covering short, medium, long, and extended wavelengths. The integral signal output of the input stage achieves a linearity of 99.7%, while the noise level remains below 0.3 mV.