The infrared radiation could be effectively attenuated by the man-made water fog.For that the water fog particles have been greatly used in the fields of infrared stealth.Some radius distribution of the water fog particles could make the stealth effect be optimal,and that has become the emphasis and difficulty of studies.Based on Mie theory,the physical and mathematic models of the water fog particles extinction were given.Three qualifications used to ascertain optimal extinction radius of the man-made water fog were summed up.In the meantime,aimed at middle and far infrared bands the optimal extinction radius of water fog particles was calculated and analyzed by compiling Matlab programs in the method of scattering phase function and extinction factor.Explicit radius distribution of the water fog particles was got,and the rule of distribution of the optimal particles radius for middle and far infrared bands was given.This conclusion could offer references for studies such as stealth of warships and land targets and fire fighting.