Mg-RE (RE: rare earth) alloys play an important role in the automobile and aerospace fields, and the microstructure of Mg alloys is a key factor in the development of innovative Mg alloys and (or) optimization of the current ones. Among different simulation methods, phase field is an effective way to predict microstructure evolution as the explicit tracking of interfaces at each time step is not required. In this work, the phase field method is employed to predict the microstructure evolution during heat-treatment of GW93(Mg-9wt.%Gd-3wt.%Y) alloy. The microstructural evolution during the solution treatment, including the Zener pinning effect and the grain growth process, are investigated. Furthermore, the volume fraction, morphology and distribution of the secondary phase under various solution treatment conditions will be investigated, and we will also give insight into the pinning effect of secondary phase on the grain boundary migration. Both the structural and concentration order parameters are introduced in accordance with kinetic and thermodynamics. Our calculation results are well consistent with experimental observations, and our calculation results may provide a theoretical support on the development of new Mg casting products.