Single-phase Mg₂Si_1-xSn_x crystal was successfully directionally solidified from the melt. Thermoelectric properties were tested for 1.5at%Bi-doped crystals with different Sn contents, and electronic transport properties were predicted by the first-principle calculation. At x=0.625, tested Seebeck coefficient and power factor is -247μVK^_-1 and 5.7mWm^_-1K^_-2, respectively, because the band structure of Mg₂Si_0.375Sn_0.625 is converged. This result is consistent with the calculated values, and the power factor enhanced 25%. The predicted and tested results of ZT maximum are 1.3 and 1.16 at T=700K, respectively. In the medium temperature range of 550K-800K, the predicted and tested ZT values can keep above 0.9. Power factor optimization is an effective way to improve the thermoelectric properties of Mg₂Si_1-xSn_x crystal. In addition, the performance deterioration of thermoelectric devices induced by nano-sized grain growth at high service temperature can be avoided.