Welding pore is the main problem during laser welding of die-cast magnesium alloys. The influences of laser power density and heat input on pore formation regularity during laser welding of die-cast magnesium alloys are studied. The formation regularities of pore are different under low and high laser power densities. Under low laser power densities (less than 1.6×106 W/cm2), porosity increases with the increase of weld heat input; under high power densities(more than 3.2×106 W/cm2), the minimum value of porosity can be obtained at certain weld heat input, and changing weld heat input a bit higher or lower than this certain value both increase porosity, but when the weld heat input is low enough, low porosity can be obtained. The different regularities can be attributed to the influences of laser power density and weld heat input on welding thermal process and the behaviors of gas sources in weld pool. It is found that suppressing the atomic hydrogen precipitation is the key of obtaining low porosity welds.