A phase-field model of ternary Mg-Gd-Y magnesium alloy was developed by coupling with the thermodynamics of Mg-Gd-Y system and considering cooling rate for the first time. It was applied to simulate the solidification microstructure and concentration distribution of GW103 (Mg-1.69mol%Gd-1.32mol%Y) alloy at different cooling rates both in one-grain and multigrain simulation cases. Then GW103 alloys were prepared via gravity casting method and characterized to verify the model. Results give new understanding that the GW103 alloy exhibits thick sixfold primary dendrite, a few protuberance-like secondary arms and even no higher-order arms, instead of developed dendrite. The ascending cooling rate results in refinement of microstructure of GW103, which exhibits smaller grain size, slimmer primary dendrite and less secondary arms in multigrain simulation case. Besides, higher cooling rate aggravates the solute enrichment and inhomogeneous distribution of Gd and Y in interdendritic area. The simulation and the experimental results are matched well.