Mg2Sn-base compounds are low-cost and environmentally friendly thermoelectric materials expected to be applied as power generators in the intermediate temperature range.Optimization of the solid solution of Mg2GexSn1-x(x=0.15, 0.2, 0.25, 0.3) compounds can be accomplished by the precise control and adjustment of the Ge content.A series of Mg2Ge0.2Sn0.8Sby (y =0.01, 0.03, 0.05, 0.07, 0.10, 0.15)compounds with excess of 10% of Mg content were synthesized by high frequency melting and spark plasma sintering technique.XRD analysis indicated that all the Mg2GexSn1-xcompounds can form solid solution for the samples with Ge content between 0.15 and 0.3.The highest power factor 4010μW/m K2 was obtained for the sample of Mg2.20Ge0.2Sn0.77Sb0.03 at 573K.The point defects, such as interstitial Mg and Ge/Sn vacancies, caused by the overstoichiometric Mg, have a positive effect on the electron concentration and thermoelectric properties of n-type Mg2Snx-based compounds.