While the car ownership increasing all over the world, the unutilized thermal energy in automobile exhaust system is gradually being realized and valued by researchers around the world for better driving energy efficiency. For the unexpected urban traffic, the frequent start and stop processes as well as the acceleration and deceleration lead to the temperature fluctuation of the exhaust gas, which means the unstable hot-end temperature of the thermoelectric module generator (TEG). By arranging the heat conduction oil circulation at the hot end, the hot-end temperature’s fluctuation of the TEG can be effectively reduced, at the expense of larger system size and additional energy supply for the circulation.This research improves the TEG hot-end temperature stability by installing solid heat capacity material(SHCM) to the area between the outer wall of the exhaust pipe and the TEG, which has the merits of simple structure, none energy consumption and light weight. Firstly, the exhaust temperature characteristics with various driving conditions are studied for the target engine. And then the candidate material and structure size of the heat capacity materials are determined according to the internal TEG thermal resistence and the heat transfer model from the exhaust gas to TEG hot-end. For various transient conditions, three materials’ specific heat capacity and thickness are considered for studying the TEG hot-end temperature stability and the average power output.The results show that the solid capacity material significantly improves the hot-end temperature’s stability of TEG but slightly reduces the average power. However, with the increase of the specific heat capacity, the high stability of the hot-end temperature and the decrease of the average power become less significant, and with the increase of the thickness, it becomes significant at a high degree.