The mechanical and thermodynamic properties of intermetallic compound in Mg-Al-Ca alloy have been investigated by Castep and Dmol program based on the density functional theory. The results show that DI3-Al4Ca is a ductility phase, on the contrary C15-Al2Ca and C14-Mg2Ca are of very good brittleness in Mg-Al-Ca alloy, and among the three phases, C15-Al2Ca is a phase with the worst plasticity. The melting point of intermetallic compound are estimated based on the calculated elastic constants; it is found that the estimated values of Al2Ca are in well agreement with the corresponding experimental values and the error relative to the experiment result is about 4.06%. By calculating the thermodynamic properties of intermetallic compound in Mg-Al-Ca alloy at different temperatures, it is found that Gibbs free energy of Al2Ca is the smallest as well as has the highest structural stability within the temperature range of 298.15-425 K, the next is Al4Ca, and the last one is Mg2Ca. In addition, with the elevated temperature, Gibbs free energy of Mg2Ca is decreased at the most rapid speed, meaning that its structural stability may be improved more rapidly as well. When the temperature is above 525 K, the case is changed; it is found that Mg2Ca has the highest structural stability, the next is Al2Ca, and the last one is Al4Ca.