The solidification microstructure of Mg-Gd-Y-Zr alloy was investigated via an experimental study and cellular automaton(CA)simulation.In this study,step-shaped castings were produced,and the temperature vari-ation inside the casting was recorded using thermocouples during the solidification process.The effects of the cooling rate and Zr content on the grain size of the Mg-Gd-Y-Zr alloy were studied.The results showed that the grain size decreased with an increase in the cooling rate and Zr content.Based on the experimental data,a quantitative model for calculating the heterogeneous nucleation rate was developed,and the model parameters were deter-mined.The evolution of the solidification microstructure was simulated using the CA method,where the quantitative nucleation model was used and a solute partition coeffi-cient was introduced to deal with the solute trapping in front of the solid-liquid(S/L)interface.The simulation results of the grain size were in good agreement with the experimental data.The simulation also showed that the fraction of the eutectics decreased with an increasing cooling rate in the range of 2.6-11.0℃·s-1,which was verified indirectly by the experimental data.