In this study, 3-mm thick low-alloy high-strength steels with different butt gaps were welded by optical fiber laser-metal inert gas (MIG) protection hybrid welding. The forming processes of weld seams with different butt gaps at a constant welding speed were studied and the formed shapes were compared with the morphology of laser-MIG hybrid welding with the same process parameters under zero gap conditions. The results demonstrate that the upper and lower widths of the weld seam and its heat-affected zone are basically same under butt gaps (U-shaped), while the weld-seam width and its heat-affected zone are up-wide and down-narrow under zero-gap (Y-shaped) conditions. The formation mechanism of weld seams is as follows: when laser-MIG hybrid welding is conducted with a butt gap, three different arc plasma shapes exist, i.e. bifurcated arc shapes formed by arc striking/arcing with either side of the test plate and the cross arc shapes formed by arc striking/arcing with both test plates at the same time. The three forms of arc shapes change continuously to form a swing arc pattern. The effect of the swing arc plasma is to preheat and melt the side wall of the workpieces. The laser heat source operates on the molten pool to increase the penetration, stabilize the arc, and eliminate incomplete fusion of the side wall caused by the arc heat source. The arc heat source with a butt gap has a large effective range, and the base metal is heated more evenly. The microstructures of the butt gap welds are more homogeneous than that of the zero-gap welding seams.