Abstract: For indoor environment of high-rise building and some special places, building material is the main radon gas source. Reducing radon exhalation rate of building material is recognized as an effective way to reduce indoor radon levels. In view of mixing a certain amount of charcoal into the concrete can reduce radon exhalation rate, the radon exhalation rates of concrete blocks with different charcoal contents (mass ratios of 0%-2%) were measured. It was found that radon exhalation rate decreases with the increase of charcoal content, and there is a clear negative correlation between them. When 1.37% charcoal was mixed, the radon exhalation rate reduced to 35% of the original exhalation rate. To study the mechanism of the reduction on the radon exhalation by mixing charcoal into concrete, porosities of those concretes were measured, but no significant difference was found. Taking other factors into consideration, it can be concluded that the adsorption of radon by charcoal is the main factor leading to the decline of radon exhalation rate. Based on the analysis and the adsorption characteristics of charcoal, an empirical formula between the relative radon exhalation rate and the content of charcoal was fitted out in combination with experimental measurement results by establishing a mathematical model and calculating.