For low speed diesel engines under severe ocean conditions, high efficiency air−water separators must be equipped to separate excess moisture contained in the air and reduce the corrosion. Optimal design of air−water separator is an indispensable part in the development of marine main engines. In view of the complex gas−liquid two-phase turbulent motion within an air−water separator with corrugated plates, a mathematical model is established and numerical simulations of flow field, droplet trajectory and secondary transport are realized. The separation efficiency and pressure drop of the air−water separator under different structure parameters and different working conditions are studied. The results show that reducing the spacing of corrugated plate is helpful to improve the separation efficiency. The bending and hydrophobic hooks of the corrugated channel are important to improve the separation efficiency. The separation of droplet is mainly concentrated on the first two stages of the air−water separator, and the separation efficiency at the third stage is significantly reduced. Research results will further support corrugated plate theory, experimental research and optimization design of similar separators.