The feature of the hot cracks of the welding joint of the MIG welded magnesium alloy AZ91D was studied systematically. The result indicates that the weld of the magnesium alloy displays a high cracking susceptibility. The cracks are mainly formed on the centerline of the weld and in the arc crater at the end of the weld. These cracks propagate along the α-Mg grain boundary, and they belong to the solidification cracking. These solidification cracks are resulted by the joint function of the low melting point liquid film in the weld and the tensile stress suffered by the weld metal during the solidification process. The low melting point liquid film is the internal cause to form the solidification cracks, while the tensile stress is a necessary condition. Limiting the amount of the low melting point eutectic and decreasing the tensile stress of the welding joint are two effective methods to improve the solidification cracking susceptibility of the Mg alloy weld.