In the background of the increasing amount of car ownership in the world, the amount of heat that is not utilized in automobile exhaust should not be ignored, and the importance of using automobile exhaust gas for power generation is gradually being realized and valued by people. However, the urban traffic environment is changeable. The frequent start and stop as well as the acceleration and deceleration of the car will lead to the fluctuation of gas temperature and flow and then the temperature fluctuation of the thermoelectric module’s hot end, which reduces the power generation efficiency of thermoelectric module. By arranging the heat conduction oil circulation at the hot end, the temperature fluctuation of the hot end of thermoelectric module can be effectively reduced, but the system volume is large, and the working fluid circulation needs additional energy supply. This research improves the temperature stability of the thermoelectric module’s hot end by installing high heat capacity materials between the outer wall of the exhaust pipe and the thermoelectric mudule, has the Advantages of simple structure and none energy consumption. The heat transfer model of exhaust to thermoelectric module was established, and experiment was carried. Under various transient conditions, we firstly studied the effects of various heat capacity materials with different thermal conductivity on the hot end temperature’s stability and average power of thermoelectric module, and than the effects of the thickness of the heat capacity materials. Based on the above analysis, we could study the hot end heat capacity’s effect on the stability of power generation of thermoelectric module. The results show that the heat capacity materials Significantly improve the hot end temperature’s stability of thermoelectric module but slightly reduce the average power. What’smore, with the increase of the heat capacity and thickness of the heat capacity materials, the increase of the stability of the hot end temperature and the decrease of the average power becomes more significant, but with the increase of the thermal conductivity, the increase and decrease becomes smaller. The results above show that the stability of power generation of the thermoelectric module improves with the increase of the hot end heat capacity.