The effect of smoke screen, whichrelative humidity; smoke screen; infrared; ultrafine graphite powders; electro-optical countermeasure is an important passive jamming means to improve the protection ability, is closely related to relative humidity (RH). The attenuation rates to 3~5 μm and 8~14 μm, settling velocities, and mass extinction coefficients of ultrafine-graphite-powder-based smoke screen under different RH are respectively measured on a humidity-controllable smoke-box experimental system, which revealed the impact of RH on the infrared interference characteristics of smoke screen. At lower RH, ultrafine-graphite-powder-based smoke screen exhibits a higher infrared attenuation rate, and a slower decrease of attenuation rate over time. With the increasing of RH, the settling velocities of smoke particles initially increase and then decrease, with a maximum settling velocity of 1.04×10-3 m·s-1 at 50% RH. The experimental results ma nifested that a decrease trend in mass extinction coefficients of ultrafine-graphite-powder-based smoke screen is associated with increased RH. The mass extinction coefficients of such smoke screen to 3~5 μm and 8~14 μm infrared at 28% RH are 2.12 m2·g-1 and 1.41 m2·g-1, which are higher than those of at 70% RH by 11.12% and 10.05%, respectively.