Laser beam welding (LBW) has emerged as one of the most promising connection techniques for lightweight equipment fabricated by magnesium alloys due to the less deformation, narrow heat-affected zone, and excellent welding efficiency. However, a variety of porosity defects are commonly found in the laser welded joints. The causes of laser-welded pores are complicated. The number, size and distribution characteristics of porosity affect the mechanical properties of magnesium alloy joints, and thus seriously affecting the reliable applications of magnesium alloy lightweight equipment. This study reviewed the mechanism of pores formation in the microstructure of laser welding joints of magnesium alloys, and discussed the influence of porosity on mechanical properties. The pore manipulation was proposed. The key basic research and applications of LBW of magnesium alloys were pointed out. This review may provide valuable insights for the development of joining and manufacturing technology of magnesium alloy lightweight parts in automotive and aerospace fields.