the invention discloses a high-toughness magnesium alloy based on 18R long-period phase ultrafine enhancement and a preparation method thereof. The high-toughness magnesium alloy the preparation method comprises the following steps of A, mixing Mg-Y-Zn alloy extruded blank is cut, then is subjected to continuous equal-channel corner extrusion processing to obtain a bulk material with a large number of twisted strips in the 18R long-period phase; B, cutting an alloy rod with the diameter of 5mm to 15 mm from the block material, wherein the length direction of the alloy rod is parallel to the extrusion direction of the equal-channel corner; and carrying out multi-pass hot-drawing processing on the alloy rod to obtain the high-toughness magnesium alloy which is ultrafine in 18R long-period phase and uniformly dispersed. According to the method, the pre-twisting of the 18R long-period phase is realized by means of equal-channel corner extrusion, the long-period phase is broken and refinedfrom the twisted strip by utilizing the subsequent large-strain multi-pass hot drawing, so that the 18R long-period phase super-refined and uniform distribution is obtained, and the strength and the plasticity of the alloy are remarkably improved.