Constant power discharging is a typical discharging mode of battery in the actual operating． The heat generated in the discharging process has a big influence on the performance and safe operation of battery． In order to study the heat generation characteristics of battery under constant power discharging，firstly，an electrochemical-thermal coupling model was established． Then，the rationality of the model was verified by experiments． Finally，the heat generation characteristics of the current collector，positive and negative electrodes，separator in a lithium-ion battery were studied． And the effects of discharging power and thickness of the positive electrode on the heat generation of each layer in the battery cell were discussed． The results show that，firstly，the heat generation rate of the negative electrode exhibits the highest degree of variation; secondly，the discharging power has little effect on the ratio of the heat generation of each layer to the total heat generation; thirdly，the heat generation ratio of the positive electrode is the largest (the ratios are all above 70% under different discharging powers); finally，the thickness increase of the positive electrode can reduce the heat generation of the battery，but this effect will de-crease as the thickness of the electrode increases．