The superplasticity of the Mg-8.59Gd-3.85Y-1.14Zn-0.49Zr alloy was investigated before and after multi-directional forging (MDF) and the mechanisms affecting superplastic deformation were analyzed. The results indicate that after MDF at a temperature of 350 °C and strain rates of 0.1 and 0.01 s^-1 (1-MDFed and 2-MDFed), the superplasticity of the alloy can be significantly improved. The elongations of the MDFed alloys exceed 400% under the strain rate of 6.06×10^-4 s^-1 and temperatures of 350, 375, and 400 °C, and reach the maximum values of 766% (1-MDFed) and 693% (2-MDFed) at 375 °C. The grain boundary sliding of the MDFed alloy is sufficient, and the energy barrier of deformation decreases. The β phase limits the grain growth and promotes dynamic recrystallization, maintaining the stability of the fine-grained structure during superplastic deformation. Several Y-rich phases nucleate in the high-strain region (i.e., the final fracture region) at high temperatures, accelerating the fracture of the specimen.