Foam interfering screens (FIS)is a new technique to solve the main problems of multi-band interfering techniques. The method to discharge FIS was given. Its optic attenuation performance was measured by the integration test system of guided missile. The result is that FIS with its size variable from 1 cm to 45 cm can make TV&IR-guided missiles invalid, and the diameter of air bladder in FIS is the only main factor to influence the attenuation effect. It shows that FIS could be effective to attenuate optical transmission in spotting and weapon guiding. And it can offer the theoretical and experimental basis for the tactics application of FIS.