Abstract: A mathematical model of two-phase flow was built in the secondary wave-type steam-water separator of AP1000. Then the two-phase flow in wave-type plate was simulated and its separation ability was analyzed. And a new type separator with relatively high separation efficiency and low flow resistance was obtained based on optimizing the structure of the old wave-type plate. Firstly, the mathematical models of steam and droplet phase motion were built in wave-type plate respectively. Secondly, the two-phase flow model was solved numerically through our own routine coupled with the software Fluent. Thirdly, the total pressure drop between the inlet and outlet of wave-type plate was calculated. Meanwhile, its separation efficiency and the moisture distributions of droplets were obtained by simulating the droplet trajectories in it. Fourthly, a new type separator with relatively high separation efficiency and low flow resistance was designed after optimizing the structure of the old wave-type plate based on separation efficiency and low pressure drop. Generally, the numerical modeling method in this paper is significantly for designing and optimizing the structure of steam separator.