Abstract: In order to prevent the indoor radon from being harmful to human health, it is necessary to accurately measure the indoor air radon concentration. Among them, the activated carbon box is one of the commonly used methods, but its accurate measurement is affected by many factors. To facilitate the accurate measurement of indoor air radon concentration, the passive activated carbon box method was used, and the γ-ray spectrum total counts of activated carbon samples with different conditions were measured by using a low background NaI(Tl) scintillation detector with lead chamber. The effects of adsorption time, dryness, dosage and position of the activated carbon on the radon adsorption capacity of activated carbon were investigated. The experimental results showed that the adsorption time was not as long as possible, the adsorption time was chosen for 3 days or 4 days for the best. The adsorbing capacity of radon was strong for the dry activated carbon, so in the first 3 days, the adsorbing capacity of radon was greater for the drying carbon than that for not-drying, and then with the adsorption time increasing, the water content for the dry activated carbon increased, and finally a phenomenon occurred that the two were equal. In the first 3 days, the penetration depth for radon in the activated carbon was less than the filling depth for the activated carbon, so the adsorption of radon was equal for the three different amounts of the activated carbon. With the adsorption time increasing, the penetration depth continued to increase, and finally the greater the amount of the activated carbon, and the greater the adsorption capacity of radon. In the closed and the radon concentration uneven indoor, the position of the activated carbon had no effect on the adsorption capacity of radon. Therefore, if using the activated carbon box method to measure the indoor air radon concentration, the dry, amount of activated carbon, and 3 days or 4 days for adsorption time should be used, and the position of the activated carbon in the indoor ground position was not to be considered.