The invention discloses a preparation method of a high-performance magnesium alloy casting, and belongs to the technical field of cast magnesium alloy heat treatment. The method comprises the following steps that firstly, hot isostatic pressing treatment is carried out on a casting prepared from a magnesium alloy with the preferable chemical components under the high temperature and high pressureconditions, and then solid solution and aging treatment are carried out under normal pressure. According to the preparation method, looseness in the magnesium alloy casting is eliminated, after hot isostatic pressing treatment and solid solution treatment, coarse eutectic phases are completely dissolved, the solute elements are uniformly distributed in a matrix, the structure is uniform, and afterthe aging treatment, the aging strengthening effect is good; and compared with a transitional preparation process of the magnesium alloy casting, the preparation method has the following advantages that the mechanical properties of the magnesium alloy casting with the preferable chemical components can be remarkably improved, and the performance uniformity, the stability and the finished productrate of the casting can be improved.