Because of their low density, high strength-to-weight ratio and high damping capacity, magnesium alloys have gained increasing applications in many fields, especially for die-cast magnesium alloys. The application of die-cast magnesium alloys in structures and repair welding of casting defects necessitates the development of welding techniques. As a high energy density thermal source, laser welding can reduce many welding defects compared with conventional welding methods, but pore formation during welding of die-cast magnesium alloys is still a severe problem that has not been solved. This paper seeks to identify the mechanism of pore formation during laser welding of die-cast magensium alloys and search for a metallurgic solution of porosity. The great tendency to pore formation in die-cast magnesium alloys is attributed to high gas content in the base metal. The characteristic of gas source in die-cast magnesium alloys and the process of pore formation during laser welding are studied by some experiments including analysis of characteristic parameters of pores (porosity, average diameter and number densities), change of densities before and after heating(350℃, holding time 1 h) and pore formation tendency under different vacuo heating conditions (heating the base metal to 350℃ under vacuum environment of different holding time). The results show that heating increases porosity, average diameter and number densities, which lead to the reduction of bulk density. Vacuo heating before welding can reduce porosity to a great extent, and with holding time prolonging, porosity reduces. In die casting, there are molecular hydrogen and atomic hydrogen in die-cast magnesium alloys. The hydrogen gas (molecular hydrogen) in high pressure pores in the base metal expands sharply in the molten pool, and the oversaturated atomic hydrogen can also form new pores or contribute to the growth of exsisting pores. That is to  say that both molecular hydrogen and atomic hydrogen contribute to the pore formation of die-cast magnesium alloys during laser welding. During vacuo heating before welding, atomic hydrogen can escape in part through diffusion, thus reducing the tendency to form pores. With holding time of vacuo heating prolonging, the amount of atomic hydrogen escaping from the base metal increases, thus porosity reduces. Adding elements, which can combine hydrogen to form hydride, may be a possible solution to pore formation in die-cast magnesium alloys. Preliminary experiments show that adding Zr to welds during welding can reduce the tendency to form pores obviously, the reason for this needs further research.