CFD based narrow gap GMAW process simulations were conducted with a swing arc model to see the influence of swing arc on the molten pool behaviors. The variations of the effective arc radius and the droplet impingement positions, which resulted from the oscillation of arc with a pulsed welding current, were considered in the model. Cases with varied arc-swing frequencies and amplitudes were studied both in simulation and experiment. The molten pool fluid flow pattern changes significantly when the arc-swing amplitude gets large, which is mainly caused by a moving droplet impingement. The low arc-swing frequency may give rise to a side-deviated weld bead due to the deflected droplet impingement. The concave molten pool surface and shorter molten pool length indicate that more heat is brought to the sidewalls by the swing arc. The molten pool oscillates constantly with high arc-swing frequency and large amplitude, which facilitates the floating up of the bubbles.