Deeper penetration depth and less pore sensitivity are the advantages of laser welding in vacuum. In order to make use of these advantages and eliminate the size limit of workpiece by the vacuum chamber as well, a new vacuum chamber is designed which can maintain a subatmospheric pressure above the welding molten pool. The chamber is fixed on the laser nozzle and the relatively lower pressure is generated with a vacuum pump. A series of spot welding and continuous welding experiments are taken under subatmospheric pressures. The results are compared with the weld bead under normal atmosphere with and without side-blowing shielding gas. The behaviors of plasma plume and molten pool are observed by the high-speed camera. The results show that the plasma plume is suppressed obviously and the penetration depth increases under subatmospheric pressure in spot laser welding. The penetration depth of spot weld bead is 4.5 mm deeper at most than the weld bead with side-blowing shielding gas. During continuous laser welding under subatmospheric pressure, the keyhole closed at the rear of the molten pool, the molten metal stacks backwards and sound weld can be achieved. The penetration depth of weld bead under subatmospheric pressure is averagely 2 mm deeper than the weld bead in normal atmosphere with side-blowing shielding gas.