Abstract: A conceptual design of a ventilation system for elimination of high radioactivity concentration radon gas from a decommisioning underground storeroom is described in this paper. A mathmatical model is constructed for predition of the variation of radon radioactivity concentration upon time in the storeroom during ventilation. The model is further improved by introduction of a modification factor M whose value is determined by fitting to the experimental data. The final model is C_t=C₀e^-(MN)t+〖SX(〗E/V+C_air•(MN)〖〗MN-λ〖SX)〗［e^-λt-e^-(MN)t］. The modified model can satisfactorily predict the radon radioactivity concentration in the storeroom at any time, and precisely calculate the ventilation time needed for decrease the radon radioactivity concentration to an acceptable level to avoid harmful inner irradiation by inhalation.