The high temperature gas cooled reactor contains a lot of carbon materials. Graphite IG-110 and boron-containing carbon (BC) are the main carbon materials. Carbon material is a porous medium, it will absorb water and other impurities. Carbon materials react with these impurities at high temperatures in HTGR. In order to reduce the corrosion of impurities on carbon materials, the HTGR core must be dehumidified before it operats. In this paper, the dehumidification of HTGR is taken as the research background. On the one hand, microscopic geometric parameters such as pore size distribution, porosity and total pore surface area of carbon materials are measured through mercury injection experiments. These parameters are the key parameters to study the macroscopic properties of materials. On the other hand, the research group transformed the original laboratory equipment into a thermogravimetric analysis device for large samples. The modified equipment can widen the volume and mass range of samples in TG experiment. In the experiment, the temperature-mass relationship of carbon materials was measured based on thermogravimetry. Six temperature points were set in the experiment. When the mass change rate of the specimen within ten minutes was less than 0.0003%, the heating furnace temperature was adjusted to the next temperature point. Finally, the mass curves of graphite IG-110 and boron-containing carbon (BC) with temperature were obtained. The results showed that boron-containing carbon (BC) could precipitate some moisture at all temperatures, while graphite IG-110 could not precipitate any moisture at 100°C.