Based on Fourier’s law and the Seebeck effect, this paper presents a mathematical model of a Thermoelectric Generator (TEG) device using the exhaust gas of vehicles as heat source. The model simulates the impact of relevant factors, including vehicles exhaust mass flow rate, temperature and mass flow rate of different types of cooling fluid, convection heat transfer coefficient, height of PN couple, the ratio of external resistance to internal resistance of the circuit on the output power and efficiency. The results show that the output power and efficiency increase significantly by changing the convection heat transfer coefficient of the high-temperature-side than that of low-temperature-side. The results also show that with variation in the height of the PN couple, the output power occur a peak value, and the peak value decreases when decreasing the thermal conductivity of the PN couple, and increases when increasing the Seebeck coefficient and electric conductivity of the material. Meanwhile, a maximum output power and efficiency of a TEG appear when external resistance is greater than internal resistance. This is different from a common circuit, and with the increment of ZT, the maximum value moves toward the direction of an increasing ratio of external resistance to internal resistance. Finally, we propose a new idea to reform our experiment design to achieve better performance.