The particles suspending in liquid suffer acoustical radiation force from ultrasonic field. The force may cause particles movement and has special advantage to separate particles from liquid in Micro Electro Mechanical Systems (MEMS). It is important to improve the energy stored in liquid layer so as to increase separation ability for decreasing drive demand in MEMS. Resonance may hugely increase the magnitude and generally be employed in micro separator. The summits of admittance spectrum show the resonance frequencies. It may come from resonance in one layer or multi layers in micro separator. This paper studies the difference of energy stored in layers at each rein frequency. It provides the way to select the most effect separation frequency other than admittance spectrum.