A mathematical model and the associated numerical technique have been developed to simulate, for the first time, the dynamic impinging process of filler droplets onto the weld pool in spot gas metal arc welding (GMAW). Filler droplets driven by gravity, electromagnetic force, and plasma arc drag force, carrying mass, momentum, and thermal energy periodically impinge onto the base metal, leading to a liquid weld puddle. Transient weld pool shape and the complicated fluid flow in the weld pool caused mainly by the combined effect of droplet impinging momentum and surface tension are calculated.