Based on scattering theory of high-energy x-ray, the structure and extinction properties of Mg2Si crystal have been studied. It is shown that for Mg2Si with antifluorite structure extinction only occurs if the indices of diffraction H, K and L are neither all odd nor all even, and diffraction occurs if H, K and L are either all odd or all even. It is of great significance to study the crystal with antifluorite structure in application.