The liquid phase of the Mg-Si binary system was re-assessed by the Calphad approach. Combined the thermodynamic data of Mg-Zn and Zn-Si binary systems, the thermodynamic parameters of Mg-Zn-Si ternary system were obtained by extrapolation. Based on the phase diagram of Mg2Si-MgZn2 pseudo-binary system, the liquid ternary interaction parameters of Mg-Zn-Si system were assessed. The results show that the calculated phase diagram agrees basically with the experimental data. The solidification process of the Mg-2.33Zn-0.9Si (at%) alloy was simulated by Scheil model, and the phase evolution of Mg alloy during cooling casting was predicted. The results agree with the previous experiment results