In the exhaust thermoelectric generator system with media fluid, the media fluid first transfers heat directly from the exhaust. Then, the media fluid is used for thermoelectric generation after heating. Therefore, the exhaust heat exchange process and the thermoelectric generation process are separate processes. A numerical simulation of this system is conducted in this paper. The results show that the output power of the thermoelectric generator system with media fluid first increases and then decreases with an increase in the module area when the exhaust heat exchange area is fixed, i.e., there is an optimal module area to maximize the output power of the system. Compared with the traditional thermoelectric generator system, for a certain condition of the exhaust heat exchange area, although the maximum output power of the novel system decreases slightly, the corresponding module area can be reduced by 74.2%. Consequently, the power generation density can reach 1254 W/m(2), which is 3.39 times higher than that of a traditional exhaust thermoelectric generator system. In addition, the temperature distribution along the module of the thermoelectric generator system with media fluid is more uniform.