Single-phase Mg₂Sn crystal was successfully directionally solidified from the melt with a high temperature gradient of 180K/cm. The critical velocity of planar interface and solidified distance for the growth of single-phase Mg₂Sn crystal is predicted theoretically and agrees well with the experimental result. The grown Mg₂Sn crystals exhibit better thermoelectric performance without influence of the second phase Sn. At the temperature ranging from 300 to 700K, the maximum Seebeck coefficient S and electrical conductivity σ reached -261μVK^_-1 and 525?^_-1m^_-1, respectively. The top value of power factor is 2.29mWm^_-1K^_-2 after the doped Bi optimization. The minimum thermal conductivity κ was measured as 4.3Wm^_-1K^_-1 at T=500K. The maximum figure of merit is 0.21 at the doping concentration of 1.5at.%Bi. The method developed in this paper can provide a methodological reference for the preparation of ternary Mg₂B^_IV solid solution.