Battery pack hierarchical cooling method and system

The system comprises an engine, an exhaust pipe, an air conditioner, a controller, a cold-heat exchanger, a radiator, a motor and the battery pack, the battery pack is arranged near the exhaust pipe and connected with the cold-heat exchanger, the cold-heat exchanger is connected with the air conditioner and the motor, the motor is further connected with the radiator, and the radiator is connected with the controller. The controller is respectively connected with the cold-heat exchanger and the battery pack; the battery pack and the motor are provided with waterways and water pumps; the controller predicts the battery pack temperature according to the engine output power; a plurality of temperature thresholds are set for the battery pack temperature grading layer, and when the temperature of the battery pack is about to exceed a certain temperature threshold, a cooling mode corresponding to the corresponding temperature threshold is started, so that the temperature of the battery pack is maintained or close to