Doping of moderate elements Sb,Te,Ag,Cu can greatly improve the thermoelectric properties of Mg₂Si. For explaning the experimental result,a calculable model is presented.It applies DV-Xα quantum chemistry computation to reveal the micro-structure information, such as the covalent bond grade and the density of state. The calculated results imply that the covalent bond grade of Mg-Si are decreased and the forbidden gaps become narrower after doping. This results appears to be consistent with the experiments.