The round-trip attenuating process of laser used for guiding by artificial water sprays is studied with the Monte-Carlo method. A new way for confirming the directions of scattered photons is established. The method is based on calculating the cumulative probability distribution function (CPDF) of polydisperse mists and user-defined function (UDF) fitting by Matlab. Software efficiencies of two kinds of Monte-Carlo methods for photons tracking are analyzed and compared. A conclusion comes to that the wight method is evidently more efficient than the event method. The radii of frequently-used water mists are usually in the range of 101～102 μm which are suited to attenuate the 10.6 μm laser and can be used for laser stealth. But the effect is worse for 1.06 μm laser, under the condition of dense water sprays, for the single scattering albedo is close to 1.