The thermal design of the lithium-ion battery energy storage system is related to the capacity, life and safety of the energy storage system. A thermal simulation method for lithium-ion battery cluster was put forward in this paper. The thermal simulation of battery cluster was divided into conjugate heat transfer simulation of battery module and flow field simulation of battery cluster. On the premise of verifying the simulation accuracy of the battery module, the simulation was used for obtaining temperature characteristic values of the battery temperature in the battery module under different coolant flow rates. Then, the coolant flow of each battery module was obtained through the simulation of the coolant flow field of the battery cluster: In the end, the difference calculation was adopted for obtaining the maximum temperature and temperature difference of the battery in the battery cluster. Such a thermal simulation method can be used for obtaining the maximum temperature and battery temperature difference in the battery cluster more precisely.