The invention provides high-strength high-plasticity cast magnesium alloy and a preparation method thereof and belongs to the technical field of composites. Aiming at solving the technical problem, the following technical scheme is adopted: the high-strength high-plasticity cast magnesium alloy comprises the following elements by weight percent: 3-6% of Zn, 8-11% of Y, 0.4-0.6% of Ca, 0.5-1.5% of Mn, less than 0.1% of impurities and the balance of Mg, and the weight ratio of Y to Zn is (1.6-2.5):1. Through reasonably controlling alloy ingredients and the melting process, high strength and plasticity can be acquired without a heat treatment process. The process is simple, the transportability is strong, and the operation is easy. The high-strength high-plasticity cast magnesium alloy can be widely applied to the fields of aerospace, rail traffic and textile technology.