Aerosols are a significant source of fission product releases from the containment to the environment during severe accidents. The migration behavior of aerosols has important implications for source term assessment in the containment and the containment design. The dominant method for removing aerosols from the containment of ACP100 is natural removal during severe accidents. The main mechanisms of aerosol natural removal include that gravitational settling, Brownian diffusion, diffusionphoresis, and thermophoresis. To study the phenomena of natural removal of aerosols in the containment during severe accidents of ACP100, an experimental facility for aerosol natural removal research was designed and constructed, and experimental studies on aerosol transient natural removal were conducted. The experimental results show that both the aerosol attenuation constant and the aerosol removal rate increase with the rate of decrease in ambient pressure. When the ambient pressure decline rate increases from 0.15 kPa/min to 0.6 kPa/min, the aerosol attenuation constant and the aerosol removal rate increase by 5.98 × 10–5 /s and 1.54 × 10–4 m/s, respectively. The rate of decline in ambient pressure contributes to the natural removal of aerosols. These results provide basic data for the improvement and full validation of the existing aerosol natural removal calculation model, supporting the evaluation of the consequences of radioactive source term assessment during severe accidents of advanced types of nuclear reactors and promoting the localization of the aerosol calculation module in the integrated software for severe accidents, which has significant practical engineering implications.