In this study, Mg-4.4Zn-0.3Zr-0.4Y (wt.%) alloy ingots were prepared by two casting processes with different cooling rates: air cooling cast (AC cast) and water cooling cast (WC cast). Then, these two types of Mg-4.4Zn-0.3Zr-0.4Y alloy ingots were extruded under the same conditions, and the effect of different cooling rates on microstructure and mechanical properties of extruded alloy was studied. The results showed that the cooling rate of melt during cast processing has a great effect on microstructure and mechanical properties of extruded alloy. Compared with AC cast alloy, the lamellar eutectic structure and dendrite cell size of WC cast alloy are significantly refined, while the formation of W-phase (Mg3Y2Zn3) was suppressed and the volume fraction of I-phase (Mg3YZn6) increased in WC cast alloy. After extrusion deformation, the WC cast Mg-4.4Zn-0.3Zr-0.4Y alloy has finer dynamically recrystallized grain size, finer and more uniformly fragmented I-phase particles distributed in matrix and grain boundaries, decreased {0002} basal texture intensity and increased {101 ?2} texture intensity than that in the extruded AC cast alloy. The yield strength and tensile strength of extruded WC cast Mg-4.4Zn-0.3Zr-0.4Y alloy reached 297.0MPa and 327.3MPa, which increased by 46.4MPa and 21.4MPa compared with that of the extruded AC cast alloy, respectively. The extruded WC cast Mg-4.4Zn-0.3Zr-0.4Y alloy showed an elongation of 14.8%, which is 4.7% improvement compared with that of the extruded AC cast alloy.