Abstract: The γ ray transport process in some pits of warhead models has been studied.Since the penetrating γ ray spectra are different for different models,the feasibility of using the γ ray from fissile materials as the fingerprint of the warhead has been analyzed.By comparing the characteristics of γ ray's intensities,the thickness of bare ball of Wg.Pu can be estimated if its thickness is less than 1.5 cm.