The synthesis of Bi dopped Mg2Si1-xSnx (0.4≦x≦0.6) solid solution thermoelectric materials with high purity was achieved by microwave-assisted solid state reaction followed by the field activated and pressure assisted synthesis (FAPAS), and its microstructure and transport properties were observed and evaluated. It indicates that the replacement of Mg used in traditional processes by MgH2 powder significantly reduces the temperature of the solid state reaction and further inhibits the oxidation and volatilization of magnesium. Meanwhile, the low-temperature and rapid heating from microwave restrain the grain growth effectively; as a result the pure product with an average grain size of 200nm has been reached. Thermoelectric performance was tested in the temperature range of 300-750K. It indicates that the solid solution with fine lamellar structure and the doping of Bi significantly lower the thermal conductivity and improve the electric properties simultaneously. The 1.5at.% Bi doped Mg2Si0.4Sn0.6 got the maximum ZT of 0.91 at 600K.