Two-dimensional(2D)transient coupled finite element model was developed to compute the temperature and stress field in cast billets,so as to predict the defects of the I-type billets made from AZ31 magnesium alloy and find the causes and solutions for surface cracks and shrinkages during direct-chill(DC)casting process.Method of equivalent specific heat was used in the heat conduction equation.The boundary and initial conditions used for primary and secondary cooling were elucidated on the basis of the heat transfer during the solidification of the billet.The temperature and the thermal-stress fields were simulated with the thermal-structural coupled module of ANSYS software.The influences of casting parameters on the distributions of temperature and stress were studied, which helped optimize the parameters(at pouring temperature of 680℃,casting speed of 2 mm/s,heat-transfer coefficient of the second cooling equals to 5 000 W/m~2 ·℃~(-1)).The simulation results of thermal stress and strain fields reveal the formation mechanism of some casting defects,which is favourable for optimizing the casting parameters and obtain high quality billets.Some measures of controlling processes were taken to prevent the defects for direct-chill casting billets.