Abstract: The relationship between the light attenuation effect and the thickness of foam/water curtain produced by foam-water dual-purpose interfere curtain of countermeasure washdown system was analyzed theoretically and studied through experiments. The curtain composed of dispersed droplets with thickness from 4 to 32 layers and different gas-liquid ratio was released to experimental deck and atmosphere, and the attenuation of parallel light was measured. Results show that attenuation of visible light by foam-water dual-purpose curtain increases significantly with thickness. The maximum of light attenuation reaches 95.8% when released to atmosphere and 98.4% when released to experimental deck. The transmission diminishes slower when the thickness increases. The research is valuable for improving interfere theory and equipment technique of foam curtain.