Abstract: A large number of carbon materials are used in high temperature gascooled reactor (HTGR), and the boron containing carbon (BC) is widely used as core structural materials because of its excellent physical properties. BC is a typical porous material, which would adsorb water vapor and other impurities when exposed to air. Its adsorption and desorption characteristics of water vapor will directly affect the dehumidification process of the reactor core. The water vapor adsorption isotherm of BC was measured in detail by dynamic adsorption and desorption experiments. The BET equation was fitted with the adsorption data of low relatively humidity, and the BET predicted value under high relatively humidity was obtained. The results show that water vapor on the surface of BC belongs to multilayer physical adsorption, and the desorption curve lags behind the adsorption curve. Water vapor can penetrate into the material, which makes it difficult to achieve the equilibrium of adsorption and desorption at high moisture content.