In the Mg-Y base rare earth alloy system, study on the diffusion growth of intermetallic compounds is important due to its close interrelation with property. Based on the experimental data of the formation and growth of intermetallic compounds ε-Mg₂₄Y₅ and δ-Mg₂Y in the Mg-Y diffuison couples, the interdiffusion coefficients of each phase in the Mg-Y binary alloy system depending on components and temperatures was calculated by numerical inverse method. The results show that the diffusion coefficient of ε-Mg₂₄Y₅ is 4 times higher that of δ-Mg₂Y. Moreover, the annealing induced element diffusion distribution, diffusion flux and growth thickness of IMC layer with time and temperature can be quantitatively simulated.