The invention provides a nanocomposite thermoelectric material and a preparation method thereof. The preparation method comprises the following steps: (1) smelting Cu, Se and La block raw materials byusing inductive smelting equipment to form a cast ingot; (2) simultaneously placing S powder proportioned and weighed according to a chemical formula and the broken cast ingot into a ball milling pot, and carrying out primary ball milling at a nitrogen atmosphere by using mechanical ball milling equipment; (3) placing a graphite die loading primary ball milled powder into a spark plasma sinteringcavity, and carrying out sintering at a vacuum atmosphere to form a block; (4) breaking the sintered block, and carrying out secondary ball milling at the nitrogen atmosphere; and (5) carrying out sintering at the vacuum atmosphere. The thermoelectric and mechanical properties of the nanocomposite amorphous/crystal Mg2Si-based thermoelectric material prepared by using the preparation method are greatly improved. According to a formula, various components are matched with each other and take a synergistic effect, so that the strength and thermoelectric properties of the nanocomposite thermoelectric material are greatly improved.