Detection of the decoys in the FOV of the imaging2infrared seeker ( IIR) is important for IIR to track the true target aircraft . It is not an easy thing when the target image only has several pixels. One of the most effective discriminations in IIR is rise2time characteristic of the decoy. A practical data smoothing method is used to eliminate the sampling effect and noise. The relative factors ,which affect the decoy detection probability ,are analyzed.