High quality Mg₃Sb₂ crystals were successfully prepared by directional solidification method. The critical velocity of planar interface of single-phase Mg₃Sb₂ crystal was predicted according to the solidification theory, and the precipitated Sb phase can be inhibited below this rate. Microstructure analysis of Mg₃Sb₂ crystals at different solidification rates indicates that, the quantity of Mg vacanices can be effectively reduced. The excess Mg atoms in the crystals is conducive to the improvement of thermoelectric performance. Carrier mobility and concentration of Mg₃Sb₂ crystal is tincreased by grain boundry eliminated and Ag doping. On the premise of keeping a high Seebeck coefficient, the maximum electric conductivity is 309Scm^_-1 at the testing temperature range of 300-800K. As a result, a better electronic transport properties of PF=1.2mWm^_-1K^_-2 is obtained. This result is verified by Hall testing and first-principle calculations. Correspondingly, the maximum ZT value is 0.67 at the doping concentration of 25at%. This method developed in this pape provides a new path for the performance optimization of Mg₃Sb₂-based thermoelectric materials, and also provides a reference for the preparation of high-performance ternary Mg₃(Sb, Bi)₂ alloy.