Abstract. In order to reveal the machining mechanism of the inter electrode gap during ultrasonic assisted electrical discharge machining, the relationship between the distribution of the flow field and bubble motion was studied. The three-dimensional model of deep hole machining was established firstly using the GAMBIT software. On the basis of theoretical analysis on the gap of the flow field working liquid, the bubble motion in the process of multiple pulse discharge was simulated using the FLUENT software. The motion of a single bubble and bubbles under with and without ultrasound was simulated respectively. Compared with the simulation results without considering ultrasound, it can be obtained that ultrasonic vibration is helpful for the generation of bubbles and improvement in the machining efficiency. It is provided a simulation basis for the actual analysis of electrical discharge machining process and the optimization of machining parameters.