The present invention relates to a fire extinguishing system, which includes, inside a box-shaped mobile loading container having a sea water feeding and discharging structure, a plurality of battery cells for supplying power to an electric propulsion ship, and thus can quickly extinguish an electric fire occurring at sea while minimizing damage caused by the electric fire. A system for extinguishing a fire in a battery loading container in an electric propulsion ship, according to the present invention, comprises: a box-shaped mobile loading container including, at one side of the upper end thereof, an inlet valve for the inflow of fire-fighting water, including, at one side of the bottom thereof, an outlet valve for the discharge of the fire-fighting water, and including a power supply device comprising a plurality of battery cells in a space formed therein; a fire detector, provided at the upper side of the inside of the loading container, for detecting a fire by measuring the temperature inside the loading container; a feeding part for feeding the fire-fighting water to the inside of the loading container by using a pump and a feeding hose connected to the inlet valve; and a control unit for controlling operations of the inlet valve, the outlet valve, and the pump according to the temperature measured by the fire detector.