Special foam screen(SFS) is a novel multi-band passive interference technique.During the processes of improving its interference effect against IR/MMW guidance by increasing the liquid-thickness of air bubbles(d),an abnormal phenomenon occurred so that the interference effect sharply declined.In order to solve this problem,the relevant phenomenon were studied.The results show that the air bubble's multi-interface characteristic is the key factor for SFS to have high effective interference,and when d exceeds dmax(the upper limit of d),the multi-interface characteristic will be severely bated and the SFS has become "degenerate SFS".The ratio of dmax to air bubbles' radius was also calculated,and it is about 0.8882.By controlling easily the working condition of the equipment to discharge SFS,the d won't exceed the dmax.These conclusions can offer the important basis for the tactics application of SFS.