The I phase (Mg3GdZn6, icosahedral quasicrystal phase) is widely considered as the strengthening phase in Mg-Zn-Gd system alloys, offering more significant improvements in the mechanical properties compared to the W phase (Mg3Gd2Zn3, cubic phase). However, both the W phase and the I phase typically coexist in the as-cast Mg-Zn-Gd alloy, thereby weakening its mechanical properties. There has been limited systematic research dedicated to investigating the crystallization mechanism of these phases during solidification. In this study, the equilibrium solidification and Scheil solidification paths of Mg-xZn-2Gd (x = 0~12, wt.%) alloys were calculated using Thermo-Calc software. The effects of cooling rate and alloy composition on the fraction of the I phase were studied. The results show that the equilibrium solidification structure of the alloy with a Zn/Gd atomic ratio of 6.0 only contains the I phase. In contrast, limited solute diffusion in the solid phase hampers the transformation of the W phase into the I phase during non-equilibrium solidification, forming a mixed structure composed of both the W phase and the I phase. The variation of cooling rate and alloy composition affects the solute enrichment rate in the Liquid during the solidification process of the primary α-Mg phase, alters the solute content and temperature of the residual Liquid when the secondary phase begins to crystallize, and influences the type and fraction of the secondary phase as determined by the solidification driving force. Higher solidification cooling rates and an increased Zn/Gd atomic ratio inhibit the W phase and promote the formation of the I phase during Mg-Zn-Gd alloy preparation, resulting in the alloy with a higher proportion of the I phase.