System and method for treating boil-off gas of ship

A boil-off gas treatment system of a ship, comprising:

• a first compressor receiving boil-off gas generated from liquefied gas stored in a storage tank of the ship and compressing the received boil-off gas;
• a second compressor compressing boil-off gas received from the storage tank;
• a heat exchanger cooling the boil-off gas compressed by the first compressor or the second compressor;
• and
• a refrigerant circulation line along which a refrigerant supplied to the heat exchanger is circulated,
• wherein the first compressor is a multistage compressor comprising a plurality of compressors and compressing the boil-off gas to a fuel supply pressure required for a propulsion engine of the ship,
• the second compressor compresses the boil-off gas to a fuel supply pressure required for a power generation engine supplied with fuel at a lower pressure than the propulsion engine, and
• boil-off gas compressed while passing through part of the first compressor is supplied to the heat exchanger to be cooled or is supplied to the power generation engine.