The purpose of this study is to investigate the numerical simulation method regarding the coalescence and breakup of droplets occurring during the gas-liquid separation process and their influence on the separation efficiency and pressure drop. The Euler-Lagrange method was used, and the discrete phase was simulated as an unsteady process. The results of the study indicate that numerical simulation results show better agreement with the experiment results when the coalescence and breakup model is taken into account. During the unsteady process, it was concluded that the simulation can meet the accuracy requirements as long as the Courant number of droplets is less than 1/3. The coalescence increases the droplet diameter, which improves the separation efficiency and reduces the pressure drop, whereas the opposite effect occurs with the breakup. Compared with other factors, the influence of the surface tension on the coalescence and breakup is more apparent, and droplets with a lower surface tension may be prone to coalescing or breaking. The coalescence occurs with a lower separation velocity, whereas breakup becomes predominant with higher separation velocity. The present research provides valuable suggestions on choosing strategies to improve the separation efficiency. For droplets with small surface tension, the separation velocity is restricted to not resulting breakup, and the separation efficiency can be improved by changing the shapes and spaces of the wave plate. In contrast, for droplets with large surface tension, increasing the velocity is an effective way to improve the separation efficiency.