The solidification behavior of AZ31 magnesium alloy plate during horizontal continuous casting (HCC) with constant velocity was analyzed by three-dimensional CFD simulation. At steady state, the temperature profile in the direction of width was slightly inhomogeneous due to wide width compared to thickness. The temperature profile in the direction of thickness was symmetrical although the temperature at lower part was slightly higher than that at upper part. The region of high temperature increased rapidly with increasing the withdrawal speed, and then the finishing point of solidification increased dramatically. The shape of temperature profile was similar regardless of melt temperature but the overall temperature of plate and the finishing point of solidification increased with increasing the melt temperature. The finishing point of solidification decreased reciprocally with the coefficient of heat transfer.