Abstract: To investigate the characteristics of the droplet phase transformation during moving in the swirl-vane separator and the influence of the droplet phase transformation on the steam-water separation performance within the steam-water separator of the AP1000 steam generator, a phase transformation model for single moving droplet was developed. The steam-water separation efficiency and the pressure drop were obtained by using the Euler-Lagrange approach based on the three-dimensional geometric model of the swirl-vane separator. The analysis of the influence of the droplet phase transformation on the separation performance and the radius variation of different sizes of droplets was conducted. The results indicate that the separation efficiency and the pressure drop increase rapidly with the steam velocity. In addition, the radius variation percentage of the smaller droplet is greater during the moving and evaporating process in the swirl-vane separator, which leads to the more likely change of the motion trajectories, the increasing probability to escape from the separator, and the smaller separation efficiency. Besides, the influence of the droplet phase transformation on the separation performance can be negligible under the actual operation conditions. The calculation results can provide theoretical basis for the further investigation of the separation performance and guide the design of the steam-water separator.