The invention relates to a nanocomposite structure Mg2Si-based thermoelectric material and a preparation method thereof, and belongs to the technical field of preparation of semiconductor thermoelectric materials. The preparation method comprises the following steps: taking Mg, Si and Sn elementary substance materials according to a stoichiometric ratio and performing high-frequency induction smelting to form cast ingot; crushing the smelted cast ingots, filling into a quartz glass tube with a lower open end, vertically placing into an induction smelting coil, vacuumizing a cavity of a rapid quenching furnace, filling protective gas, performing induction smelting to enable the block to reach a molten state, spraying the melt to a copper rod, throwing out to form belt materials, and collecting the belt materials; placing the belt materials into a glove box under the argon protective atmosphere, grinding into powder, and performing spark plasma sintering into blocks. The preparation method is simple and feasible; the process flow is short; oxidation of Mg can be effectively inhibited; process parameters are easy to control. An amorphous / nanocrystalline composite structure exists in a sample, the grain size is obviously refined, the grain size distribution is controllable, scattering of electrons and phonons is increased, the Seebeck coefficient is greatly increased, and the thermoelectric property of the material is improved.