In this research, the Mg2Si1-xSnx thermoelectric material is used in the exhaust temperature difference power-generating system, and the material's heat transfer characteristic and power-generating characteristic were analyzed. Firstly, steady heat transfer model from vehicle exhaust to cooling water was established. Then the impact of Sn/Si ratio to the thermoelectric characteristic parameter was analyzed. Finally, considering the influence of varying thermal conductivity to the heat transfer process along the material's heat transfer direction, when the cold end temperature of thermoelectric materials was controlled by cooling water respectively boiling at 343K and 373K, the thermoelectric conversion efficiency and power output of Mg2Si1-xSnx thermoelectric materials with different x value were evaluated based on simulation calculation.Results show when the engine's rotation speed is in the range of 1700RPM to 2100RPM, using 373K water boiling control, the best Sn/Si ratio is x=0.7, and the biggest output power of 104 modules was 3050W. When cooling water boiling at 343K, the best Sn/Si ratio is x=0.7, and the biggest output power of 104 modules was 3450W. Compared to the result of water boiling at 373K, the increase of maximum power output was 350W when cooling water boiling at 343K, which proves that water boiling at 343K would make power generating system gain better thermoelectric performance.