Increases in the penetration depth of laser welding has gained undoubted interest, especially in the aerospace, power station, ship building, and other heavy industries. Gas-jet-assisted keyhole laser welding is a prospective method for improving the penetration of conventional laser welding. A series of experiments using this method were conducted with different parameters of the assisted gas jet and the welding speed. The microstructures of weld joints were observed using optical microscopy, and microhardness was also measured. The investigation results showed that the penetration depth of this laser welding increased by more than 20%, with a maximum increase of approximately 26%, at different welding speeds, while the weld width was significantly reduced compared with that of conventional laser welding. The key factor affecting the penetration increase is the interaction between the assisted gas jet and the plasma. The penetration increase was determined by the distribution and amplitude of the assisted gas jet at the position of the keyhole orifice. The grain in the heat-affected zone (HAZ) and weld seam of gas-jet-assisted keyhole laser welding was finer, and the number of columnar grains was also significantly reduced. The microhardness of the HAZ for the assisted gas jet was much lower, and more pearlite and less martensite were observed this zone. This was caused by the reduced maximum temperature of the molten pool, reduced high-temperature residence time, increased cooling rate, and diminished temperature gradient with the introduction of the assisted gas jet.