The invention relates to a high-strength magnesium alloy and a preparation method thereof, in particular to an enhanced high-strength magnesium-zinc-yttrium (Mg-Zn-Y) alloy capable of precipitating small similarly-spherical quasi-crystal phase through solution and aging. The mass percent of chemical elements in the alloy is that: 5 to 6 percent of Zn, 1 percent of Y, Mg supplementing the percentage to 100 percent, and the mass ratio of the Zn and Y falls in between 5 and 6. Zn is added in a form of pure Zn, Y is added in a Mg-Y master-alloy form with Y content of 25 percent by weight, and Mg adopts the industrial pure magnesium. The alloy prepared through preheating, smelting, refining, metal forming and heat treatment consists of alpha-Mg solid melt and icosahedron quasi-crystal phase, the shape of the quasi-crystal phase is similar to a spherical shape, and the size is small. The precipitated quasi-crystal particles are propped against dislocated intermediate pinning and a firm interfaces with a substrate, so that the magnesium alloy can be remarkably strengthened. The tensile strength of the high-strength magnesium alloy is more than 250MPa, and the elongation rate is more than5 percent. Due to the adoption of the quasi-crystal enhanced high-strength Mg-Zn-Y alloy and the preparation method thereof, the high-strength magnesium can be widely popularized to use in fields such as aviation, aerospace, traffics and the like.