Abstract: Based on the first-principles methods,the lattice parameter,equation of states and phonon dispersion curves of the three Mg2Si polymorphs are calculated by using the plane-wave pseudo-potential method.The anti-fluorite structure,the anti-cotunnite structure and the Ni2In-type structure of Mg2Si can retain their mechanical stability in the pressure intervals 06 GPa,7.520.2 GPa and 21.940 GPa,separately.The mechanical stabilities of three Mg2Si are: anti-fluorite s.