When detecting ballistic midcourse target with the Space-based Infrared System-low(SBIRS-Low), the background radiation of atmosphere or stellar is usually much stronger than that of target. Detection midcourse target is difficult. Based on design requirements of space-based infrared system-low, a method of detecting ballistic midcourse target with the space-based multi-band image system is inversed. In general, endmembers of target and atmosphere background are uncertain, and endmember of stellar radiation can been obtained by estimating the atmosphere absorbance. In this condition, a new algorithm based on OSP algorithm is proposed. Background endmembers matrix is made up of stellar endmember and atmosphere endmembers extracted with PCA method. Target endmembers is replaced with a unit vector. Computer simulations results show that the algorithm can effective suppress the background clutter of stellar and atmosphere. And midcourse target can been detected reliable.