Abstract: Based on theories of fluid mechanics and electromagnetics, the mathematical model of the transition behavior in ultrasonic assisted metal inert gas welding（ultrasonic-MIG） was established by adding ultrasonic vibration and radiation force conditions. FLUNET software was used to simulate the droplet growth, deformation and shedding under different welding currents and ultrasonic amplitudes. The simulation results showed that the droplet profile change and shedding time were in good agreement with the related test results. Compared with conventional MIG, the drop size of ultrasonic-MIG droplet was smaller and the transition frequency was faster under the condition of low current,while the drop time and size of ultrasonic-MIG droplet changed less obviously under the condition of high current. The dropping time of ultrasonic-MIG droplet was affected by ultrasonic amplitude, and the increase of ultrasonic amplitude could shorten the droplet transition period to some extent.