The invention discloses a high-strength and high-toughness Mg-Zn-Y-Mn-Mo-B alloy and a preparation method, and belongs to the field of alloy preparation. The preparation method comprises the steps that firstly, pure magnesium is added into a crucible preheated to 400 DEG C, and after the pure magnesium is molten, other raw materials are sequentially added into the crucible; secondly, molten alloymelt is poured into an ingot; and thirdly, the casting ingot is subjected to two-stage homogenization treatment and then extruded into bars. According to the high-strength and high-toughness Mg-Zn-Y-Mn-Mo-B alloy and the preparation method, through the microalloying effect of Mo and B, the multi-phase composite synergetic enhanced fine grain structure and the high strength and toughness and corrosion resistance are obtained, the tensile strength reaches 430-460 MPa, the elongation reaches up to 22%-25%, the corrosion potential of the alloy ranges from E=-1.386+/-0.3 to E=1.470+/-0.3, the corrosion current density ranges from I=17.89+/-0.3 to I=25.89+/-4.0, and the good corrosion resistance is achieved.