An ultra-high strength and toughness as-cast Mg-10Gd-1.7Y-1Zn-0.5Zr (wt.%) alloy was prepared via special ultrasonic melt treatment, and the peak-aged (200 ℃ for 48 h) ultimate tensile strength (UTS), yield strength (YS) and elongation (EL) at room temperature reaches 430 MPa, 324 MPa, and 13.6%, respectively. The ultrasonic treatment during semi-solid conditions refines the grains and hinders the growth of the divorced eutectic phases (α-Mg + Mg₃Gd) during casting. During the solution treatment, the refined Mg₃Gd phase suppresses the formation of cubic-shaped GdY phases and block 14H long period stacking structure (14H-LPSO) phases, and further increases solute concentration in the matrix. More solute atoms promote the growth of β’ plates with an increased aspect ratio of 8:1, which can increase critical resolved shear stress (CRSS), YS and work hardening rate. It is that found the aspect ratio of β’ plates are mainly responsible for the improvement in the strength of the alloy under the same composition and heat treatment conditions.