In order to develop a new method for predicting the internal crack in DC casting billet, particular tensile tests during the mushy zone of high strength Al-Mg-Si alloys and solidification and thermal stress analysis in the DC casting process of the billet were performed. In this study, the solidification analysis was calculated using a commercial solidification package, CAPFLOW and the thermal stress analysis was calculated by a structural analysis package, ANSYS. The thermal histories from the CAPFLOW were used as input data to an elasto-plasticity model which simulated the thermal stress and deformation of the billet during the casting process. By comparing the equivalent plastic strain obtained from the elasto-plasticity thermal deformation analysis with the break-distortion of the alloy obtained by high temperature tensile tests, the prediction of the internal crack in DC billet became possible. The analysis results in mush zone show that the equivalent plastic strain is an important factor for the occurrence of internal crack. When the equivalent plastic strain exceeded the break-distortion of the alloy, the billet will be cracked, and the results of prediction are in good agreement with the experimental results.