To investigate the influence of fluid characteristics on the polishing properties during sono-assisted polishing and analyze the affecting pattern of ultrasonic wave on the material removal rate(MRR)at the workpiece surface, for smooth polishing pad and pad with small holes, simulation models for ultrasonic vibration assisted polishing under different film-thickness are built and analyzed with FLUENT software. The influence of different process parameter on the value and distribution pattern of pressure, velocity and air phase are obtained. Simulation results show that the ultrasonic wave can cause strong shear flow and high pressure at the workpiece surface, and the shear flow and pressure grow stronger with the decrease of film thickness. Pores at the pad break the continuous fluid flow under workpiece, which makes velocity and pressure of fluid change abruptly and creates more ultrasonic cavitation, which increases the MRR.