Abstract: Recently, nature radiation has got more attention, also including the impact of radon and its daughters. The change rate of radon concentration relates to radon exhalation rates and back-diffusion in a closed space. But now, the researches on the radon exhalation rate are much more than the back-diffusion coefficient. We discussed the theoretical equation of the concentration of radon in a closed space and the influence of back-diffusion. In this paper, We detected the concentration of radon in a closed space. Based on the experimental data, the characteristics of back-diffusion are discussed. By comparing the theoretical equation with the measured data, the relationship between the radon concentration and the precipitation rate and the back-diffusion coefficient is verified. The results provide a reference for the analysis and estimation of radon concentration in closed spaces such as indoors or mines.