The invention discloses a preparation method of ultra-thin grain magnesium alloy extruded bars. In a process of preparing a magnesium alloy bar by hot extrusion, while being extruded, magnesium alloyis twisted synchronously, thus, the magnesium alloy deform violently under the effect of a composite stress field formed by tensile stress and torsional stress, the structure of the magnesium alloy isrefined effectively, and the comprehensive mechanical property of the deformed magnesium alloy is improved. By the method, a purpose of implementing grain refinement is achieved by a conventional process, low-cost and high-performance deformed magnesium alloy is developed, the deformed magnesium alloy product can be produced by using simple equipment fully, a preparation process is shortened effectively, energy is saved, the cost is reduced, and popularization and application are facilitated.