The invention provides a Mg-Al base multi-element magnesium alloy and a sub-rapid solidification preparation method of the Mg-Al base multi-element magnesium alloy, in particular to a novel high-performance sub-rapid solidification magnesium alloy. According to the sub-rapid solidification preparation method, through multi-element alloying and a sub-rapid solidification means, grain is refined, an eutectic phase is coarsened, composition segregation is relieved, then high-solid-solubility solidification structure is obtained, and the solid solution treatment time is shortened; by means of follow-up rolling treatment, high-solid-solubility solute atoms in the sub-rapid solidification process are evenly separated out, so that a deformed structure with fine grain and a dispersed second phaseis formed, and then a Mg-Al base multi-element magnesium alloy rolled plate with excellent mechanical property is obtained. The sub-rapid solidification preparation method is particularly suitable for the Mg-Al base multi-element alloy high in aluminum content, simplifies the preparation process of the magnesium alloy rolled plate and provides an effective approach to the improvement of the deformation capacity of the magnesium alloy and promotion of industrialization of the magnesium alloy plates. The Mg-Al base multi-element magnesium alloy mainly comprises the chemical ingredients including, by mass, 5.5-6.4% of Al, 0.5-2.0% of Zn, 0.5-2.0% of Sn, 0.2-1.0% of Bi, 0.1-0.5% of Mn and the balance Mg.