The valence electron structures of the matrix and the precipitated phases of Mg17Al12 and Mg2Si of Mg-Al-Si alloy were calculated, then the space structures of covalent bonds of precipitated phases were analysed. The results show that the structures of the two main bonds of Mg17Al12 are connected through the weaker bond G and its thermostability are lower. The strongest bond A of Mg2Si has three dimensional structure and does not exist the weaker segment of covalent bond structure, so its thermostability is high. Under room temperature, the strengthening mechanism of Mg17Al12 and Mg2Si lies in their atom bond force far large than that of the matrix and they effective locate the dislocation movement. When temperature is above 120 °C, the bond G breaks and Mg17Al12 disrupts, so it can not play a role of locating the grain boundary and controlling the grain boundary of high temperature. But it is not easy for the covalent bond structure of Mg2Si to disrupt under high temperature, so it can effectively obstruct the movement of the grain boundary and dislocation.