The solution redistribution was an important phenomenon during the solidification of multi-component alloys. The changing disciplines during solidification of different component Al-Si-Mg alloys were calculated in this paper. The calculations were coupled with CALPHAD technology. The interaction of solutes would change the solute redistribution coefficients during the solidification especially in the ends of solidification. So in the ends of the solidification, the slope of the curves turned to bigger and bigger. The results of the calculating of the eutectic fraction of the alloys show that errors exist under assuming the partition coefficients of solutes as a constant due to the interaction between solutes in ternary alloys. The predicted eutectic fractions of Al-Si-Mg alloys agree well with the experimental results for using the CALPHAD methods.