The present disclosure provides a battery pack comprising: a module assembly having a structure in which a plurality of battery modules is mounted on a cooling plate having an internal hollow structure capable of accommodating a fluid; a pack housing having an inwardly recessed seating portion for mounting of the module assembly, and having an opened upper end; and a coolant circulation system configured to supply a liquid coolant into an internal hollow of the cooling plate to absorb heat of the battery module mounted on the cooling plate, wherein the pack housing includes a drain hose to discharge the liquid coolant that flows out from the coolant circulation system to the outside of the pack housing, and an opening through which the drain hose is mounted is formed on one surface of the pack housing.