The foam can attenuate the electromagnetic radiation by scattering，reflection and absorption， this mechanism is discussed. The target discovery and identification probability of infrared thermal imager in the ideal and natural conditions are analyzed. Based on the conclusion and Lamber-Beer law，the mathematical model of the target discovery and identification probability of infrared thermal imager in the foam attenuation masking condition is established. The values are calculated by using Matlab simulation software. The conclusion shows that the target discovery and identification probability of infrared thermal imager are decreased，with increasing of foam thickness，and that is consistent with the experimental values.