Abstract: The theory analysis and three-dimensional numerical simulations were carried out to study the re-entrainment mechanism in a single-hooked wave-plate separator under operating conditions. The Realizable k-ε model was utilized to calculate the steam flow fields of the separator. The discrete phase model with eddy-interaction model was adopted to predict the trajectories of the droplets. The equations of the film motion were solved to obtain the film velocity and thickness distribution. Four potential regimes of secondary droplet formation were analyzed theoretically. The results show the possibility of droplet breakup by aerodynamics and splashing by droplet impinging the film is low, but higher than that of the cold conditions. The film exists mainly on the first two stages and thickens and moves downwards with increasing the steam velocity or wetness at the separator inlet. The film stripping and separation are proved to be the main reasons for the re-entrainment and unified with one discriminant. Compared with the film stripping, the film separation at the peak of the hook occurs more easily.