The invention discloses a nano-composite structured Mg<2>Ge/Mg<3>Sb<2> thermoelectric material and a preparation method therefor. The thermoelectric material consists of an Mg<3>Sb<2> base phase and an Mg<2>Ge nano phase, wherein the Mg<2>Ge nano phase is dispersed in the Mg<3>Sb<2> base phase; and the Mg<2>Ge is 10-20nm in dimensions. The thermoelectric material is prepared by the steps of fusing, and melt rotating and throwing by combination with spark plasma sintering; and the obtained thermoelectric material belongs to a p type thermoelectric material, so that phonon scattering is improved, thermal conductivity is greatly reduced, the Seebeck coefficient is improved, and relatively high thermoelectric performance is achieved.