To explore the mechanism of ultrasonic vibration assisted polishing, the effects of different ultrasonic vibration parameters on the flow field parameters, e.g., absolute pressure, velocity and vapour volume fraction, have been analyzed by using FLUENT unsteady dynamic grid turbulence and discrete phase model combined with cavitation module of mixture. The inherent rules and relations of these parameters have been investigated to reveal the phenomenon of secondary cavitation. The simulation results show that the effect of ultrasonic cavitation is mainly concentrated on the small area just below the specimen, so there is an effective processing area in the flow field, the minimum film thickness required for polishing effectiveness of the flow field can be determined. There are two peaks of the absolute pressure, velocity, vapour volume fraction within a vibration cycle which is related to the vapour bubble grow and collapse, namely, the secondary flow field cavitation phenomenon, which enhances the erosion effect during polishing process. There are time-lags between these fluid parameters at same position and ultrasonic vibration parameters, as well as for the same flow parameter at different position in the flow domain. The ultrasonic vibration results in transverse wave in the fluid, after meeting the reflected one, mechanical wave interferes and half wave loss occurs, which will affect the effective processing area. When the standing wave appears at condition of large-enough film thickness, the vibration amplitude of the wafer surface is increased and polishing effectiveness can be greatly improved. So the wafer processing can be optimized by determining the effective processing area and utilizing standing wave phenomenon. The secondary cavitation and time-lag phenomena are helpful in revealing the mechanisms of ultrasonic-assisted polishing(UAP).