A structure model of Micro Electro Mechanical System (MEMS) ultrasonic separator which based on the two dimensional normal vibration modes was chosen as the research object in this paper. Firstly,in accordance with the theory of electricity sound analogy,a two dimensional equivalent circuit model of the rectangular section of the fluid cavity was figured out.According to this model, the resonance frequencies of the two dimensional normal mode were listed.Secondly,Simulations using finite element method with ANSYS software was used to demonstrate the validity of the model and helped to select the best exciting frequencies;in the end the frequencies of two dimensional normal vibration modes(2,1) and (1,1) were chosen in this paper to excite the ultrasonic.The results showed good separation.