The invention discloses a fire extinguishing system using a power battery liquid cooling device. The fire extinguishing system comprises a battery cell module, a temperature sensor, a gas sensor, a smoke detector, a flame detector, a water cooling pipeline, a cooling liquid nozzle, a cooling liquid injection valve, a fire extinguishing agent storage tank, a fire extinguishing management system, analarm display screen, a connecting wire harness and a battery management system, wherein the temperature sensor is arranged in the battery cell module; the flame detector which is in electric signalconnection with the fire extinguishing management system through the connecting wire harness is arranged on the outer side of the battery cell module; the gas sensor, the smoke detector and the fire extinguishing agent storage tank communicate with the fire extinguishing management system through the connecting wire harness; and the water cooling pipeline is further arranged on the outer side of the battery cell module. Through the application of a proper control strategy, when a battery is subjected to thermal runaway or combustion, cooling liquid in a liquid cooling system is controlled to be injected in a directional mode, the advantage that the specific heat capacity of the liquid is large is utilized, the temperature of a battery cell is rapidly reduced, and the thermal runaway or flame spreading speed is delayed to provide enough escape time for passengers.