Radon is a natural radioactive noble gas that is naturally found in the ground. Exposure (excessive) to radon can pose a threat to the public health and get the public attention wordwide. Requiring the balanced mixing of radon and its daughters in indoor radon and outdoor air, a quantitative calculation model is deduced. The results showed that the indoor radon levels reach a steady value with the increases of ventilation radio and time, resulting in the balance for radon concentration indoor and outdoor air ultimately. Based on the model calculation, radon in door air has a theoretiacl value ranging between 6.35 and 40 Bq/m 3, which are in accordance with the actual measurements（6.0～50 Bq/m 3）by Tianshan Ren and Mei Wang, etc. The established quantitatively model for calculating the change of radon levels in indoor air could provide the technical support for indoor radon control and environmental assessment.