The second phase in Mg-Zn-Y alloy was successfully extracted by electrochemical phase separation, and the solid solution amount of alloy elements in Mg-Zn-Y alloy was obtained. A method for direct determination of solid solution in magnesium alloy was established. The activity and activity interaction coefficients of Zn in Mg-Zn-Y ternary alloy were calculated by using Miedema mixed enthalpy model and Toop model (symmetry) and Kohler model (asymmetric). The experimental measurements and calculations show that the interaction coefficients of Zn and Y in Mg-Zn-Y alloy are positive, which shows that Zn and Zn, Zn and Y are mutually exclusive in this system. The existence of Y can improve the activity of Zn, that is, the increase of Y solution will lead to the decrease of the solid solution amount of Zn. Thus, the supercooled degree of the liquid phase in the front of the solid-liquid interface is increased, so that the grain is refined, and the Mg₃Y₂Zn₃ eutectic phase of the grain boundary is increased and the mesh is gradually continuous.