Abstract: When aluminum alloy was welded by metal inert-gas(MIG) welding with longitudinal intermittent alternative magnetic fields, a highspeed video camera was used to view the whole process of electrode melting, a droplet developing at the electrode end, the droplet detaching and transferring to the weld pool.And the difference between droplet transition with longitudinal magnetic and without longitudinal magnetic was analyzed.The experiment results indicate that there are obvious change in the metal transfer with longitudinal intermittent alternative magnetic fields, which the shape of droplet changes from globular without longitudinal magnetic to flat shape, and the droplets detach from the tip of the wire, diverge from the axial line of the wire and fall down with turning movement.With the same frequencies, droplet transition time ranges from 4.5 ms to 6 ms when excitatory current is smaller, but droplet transition time needed ranges from 5 ms to 15 ms when excitatory current is larger, so excitatory current has a large influence on droplet transfer.