A magnesium cylinder head cover was used to experimentally study the influence of application of decreasing speed technology(denoted as DST) and decreasing pressure technology(denoted as DPT) on pressure variations in the die cavity, the surface quality, properties and gas porosity of magnesium alloy die castings. The results show that the application of DST and DPT during mold filling of magnesium die casting leads to the decrease of filling pressure and the decrease of surface quality of the castings. Application of DST or DPT respectively causes the decrease of the pressure during pressure intensification stage and a little increase of the number and size of gas porosity in the castings. As a result, the tensile strength and density of the castings decrease. Application of DST and DPT simultaneously causes a great decrease of filling pressure and the pressure during pressure intensification stage and a sharp increase of the number and size of gas porosity in the castings. Accordingly, the tensile strength and density of the castings decrease greatly.