This paper investigates the effects of Y content on the microstructure and mechanical properties of Mg-9Gd-xY-0.5Zr alloys. The mechanical properties, together with the grain size, first increase and then decrease, exhibiting a non-monotonous change with the increase in Y content. The alloy with 3 wt.% Y exhibits the best mechanical properties compared to the Y free alloy, with an increase in ultimate tensile strength and yield strength of 33.4% and 19.2%, respectively. However, further increase in the Y content (5 wt.%) does not enhance the strength but promotes the growth of the Mg₅(Gd,Y), becoming nucleation points and propagation paths for cracks, leading to a decrease in performance. The cubic phase REH₂ and oxide are not the reasons for the poor ductility in as-cast Mg-9Gd-xY-0.5Zr alloys due to low content, and the crystallographic orientation relationship between REH₂ and α-Mg is (220)_REH2//(11$\overline{2}$0)_Mg, and the interface mismatch is 9.13%. This paper systematically prepares and investigates the high-strength as-cast Mg-Gd-Y-Zr alloy, which has important guiding significance.