Abstract: Problems about dynamic force equilibrium on liquid droplet during arc period of short-circuiting transfer are theoretically approached.Dynamic electro-magnetic force and its influence on the stability of short-circuiting transfer during arc period of short-circuiting transfer are mainly discussed.The results show that the dynamic change of electro-magnetic force on droplet relates to the variations of droplet size,arc root position and welding current.If dynamic force equilibrium on droplet during arc period of short-circuiting transfer can not be maintained,free-flying transfer of droplet will occur.