Foam is a new way of passive optic-electronic jamming.Its jamming effect on laser guided weapon system is obvious.In this paper foam jamming technology is summarized at first.Then the composition and working principle of laser guided weapon are introduced briefly.According to the requirements of target detection of laser guided weapon，the sheltered surface density of laser guided weapon and foam surface density between two points are calculated respectively.By comparing two surface density,jamming effectiveness of foam is analyzed.The conclusions of this paper have important value for foam jamming efficiency evaluation and optimization of foams jamming system.