Abstract: The research was carried out on the variation of the radon exhalation rate of aerated concrete block with the water content and dimension by using the RAD7 radon monitor and the radon exhalation rate test cabin of building materials. The result indicates that the amount of the same samples has no influence on the samples’ radon exhalation; if the volume of the aerated concrete block in the test cabin is unchanged and the surface area increases, the radon exhalation rate decreases, but the total of the radon exhaling from the surface of the aerated concrete block keeps constant; for the aerated concrete block, its radon exhalation rate increases with water content as logarithmic growth.