Abstract: High-temperature gas-cooled reactor (HTGR) is the first kind of power plant with fourth-generation features in the world, and a large amount of nuclear graphite and carbon material is adopted as structural material and fuel matrix material. The carbon material is porous material, and absorbs moisture and other impurities from air atmosphere. The dehumidifying process of initial reactor core or post-accident core was determined by the moisture diffusion. The time-moisture absorption capacity curve, which was obtained from the dynamic moisture absorption experiment on carbon material with boron carbide (BC), was fitted with simulation result to derive the effective moisture diffusion coefficient D_e in BC, and D_e is 7.30×10^-10 m²/s. The influence of humidity and temperature on equilibrium moisture adsorbance was introduced. The equilibrium moisture adsorbance increases linearly with the environmental humidity, and decreases with the increase of temperature.