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The semi-solid Mg-Gd-Ca-Zr alloys slurries with fine and spherical α-Mg particles were successfully prepared. The influence of solute Gd and Zr contents, and cooling rate on the morphological evolution of α-Mg are systematically investigated. The results showed that addition of Zr will turn the coarse dendritic α-Mg particles into fine and globular particles. With the increase of Zr addition from 0 to 0.6 wt.%, the particle size decreases, and the particle density and shape factor increase continuously. With the change of solute Gd content from 5 to 15 wt.% in the Mg-yGd-2Ca-0.6Zr alloy, the particle size of α-Mg particles decreases continuously, and shape factor increases continuously. With cooling rate decreases from 3 to 1.5 °C/min, the α-Mg particles are slightly coarsened and the morphology are kept nearly globular, while with further decreasing into 0.5 °C/min, the α-Mg particles are severely coarsened and turned into irregular shape. The optimal Zr addition is ∼0.6 wt.% and Gd addition is no less than 10 wt.%, and the optimal cooling rate is 1.5 ∼ 3 °C/min. The mechanisms for obtaining fine and spherical α-Mg particles through Zr and Gd additions are discussed to be different roles of heterogenous nucleation effects and growth restriction effects.