Gas cyclones are widely used to purify waste gases but are not effective in collecting fine particles. The cloud-airpurifying (CAP) technology was recently developed to overcome this deficiency by unitizing the supersaturated vapor in the cyclone. This paper presents a CFD study on the CAP cyclone. The condensational growth of particles is implemented in the CFD model. its positive effect on the collection efficiency is confirmed and more dearly understood from microscopic investigations. Particles with the same initial size can grow into a narrow size distribution, yet the average growth ratio is increased with the increase of the supersaturation ratio and the decrease of the initial size, resulting in a significant improvement of the collection efficiency for fine particles, particularly in controlling those escaping through the middle-reverse flow. A predictive model is developed to consider the effect of the vapor. These results can help understand and optimize the CAP cyclone. (C) 2020 Elsevier B.V. All rights reserved.