Abstract: In the event of a reactor severe accident, the hygroscopic aerosols released from the reactor core will grow in the moist containment, thereby affecting their natural removal process. In this study, a growth model of the hygroscopic aerosols was developed and it was also validated in a variety of ways. The model calculation results shows that there is a humidity threshold in the growth progress of the aerosol particles due to the limit of the solubility and the aerosol particles do not absorb the water below this threshold, which is ignored by other severe accident analysis programs. Based on the severe accident condition of a third generation advanced pressurized reactor, the effects of the dry particle radius and the humidity on the aerosol particle equilibrium radius and natural removal coefficient were also investigated. The results show that the natural removal coefficient of aerosol particles decreases firstly and then increases with the increasing of the dry particle radius, and it takes the minimum value at 1 μm. At the same humidity, the dry particle radius has little effect on the max growth ratio of aerosol particles. The effects on the removal coefficients of aerosol particles with different dry particle radius are different.