Due to obvious difference of background, transient lost of target lose or intensive displacement of frames of target when infrared system in the sky observing targets in midcourse trajectory of ballistic missile. Therefore, with combination of moment invariant in pattern recognition and the target's characteristics, possible star and warhead target are selected through classifying three times. This method not only realizes steady detection of warhead among consecutive changing frames, but also guarantees the solidity of warhead target detecting algorithm as the background changes suddenly among frames and star images draw out.