The invention discloses an Mg based metallic glass film and its preparation method. The chemical general formula of the film is MgaCubYc, wherein a, b, and c are atomic percentage, a is greater than or equal to 46.30 and smaller than or equal to 58.01, b is greater than or equal to 24.61 and smaller than or equal to 28.96, c is greater than or equal to 13.18 and smaller than or equal to 24.74, and the sum of a, b and c is 100. The invention also discloses a preparation method of the Mg based metallic glass film, which is obtained by magnetron sputtering. And the method includes: preparing Mg, Cu and Y into an alloy target according to certain proportion; using a monocrystalline silicon wafer and an ordinary glass sheet as the film substrate; putting the alloy target and the film substrate into a sputtering chamber, and performing magnetron sputtering, thus obtaining the Mg based metallic glass film. The method of preparing the metallic glass film by magnetron sputtering only needs ordinary water cooling to obtain an ultrahigh cooling rate, the amorphous is easy to form, and by means of single target sputtering, the film components and thickness are adjustable, and the preparation cost is low. With large differences in amorphous and crystalline resistivity as well as reflectivity, the Mg based metallic glass film has great application prospects in phase-change optical disks and phase change memory.