The invention relates to a method for preparing high strength and high plasticity aluminum-magnesium alloy of a mixcrystal structure. The preparing method includes the two steps of homogenizing treatment and intermediate annealing-assisted large reduction controlled rolling. The method specially comprises the steps that element macro segregation in a cast-state structure is removed through high-temperature homogenizing treatment at first to achieve high alloying element solubility; and then controlled rolling deformation is combined with intermediate annealing treatment to introduce high-density dislocation, and recrystallization is promoted to refine crystal grains. Nano-grains, ultra-fine grains and micro-grains in the aluminum-magnesium alloy structure prepared through the method are mixed to coexist, and compared with a single nano-grain or ultra-fine grain structure, the mixcrystal structure aluminum-magnesium alloy has high strength and high plasticity. Dislocation motion, accumulation and storage in the plastic deformation process can be facilitated through the mixcrystal structure, plastic strain local concentration of a nano-grain metal material is effectively prevented, and work hardening and plastic deformability of the material can be improved obviously.