Abstract: The computational fluid dynamics technique was adopted to calculate and analyze the influence of droplet size on the swirl vane separator performance and internal flow details. The Euler two-fluid model with the mean particle size method was utilized to calculate the two-phase flow in the separator, with the particle size in the range of 0.1-150 μm. By analyzing the flow field details, the internal relations were revealed between the particle size and the separation performance. The results show that the pressure loss declines after the first rise with the increase of particle size, while the separation efficiency has an “S” type increasing trend. The particle size having impact on the separation efficiency is in the range of 5-150 μm, while that of the pressure loss is in the range of greater than 5 μm. Compared with the experimental results, the calculated pressure loss has a relative deviation within 4.8%, and the appropriate choice of the particle size contributes to a more accurate separation efficiency and outlet humidity, and the calculation method is proved to be reliable. The steam flow distribution ratio and water reflux ratio decrease with the increase of particle size, while the ratio of the reflux water accounting for the total amount of water is high, up to 48 %, and therefore it must be considered in the calculation, and the peripheral space is proposed to be considered in the computational domain.