An energy saving system using waste heat of a ship is disclosed. The energy saving system using waste heat of a ship of the present invention comprises a waste heat recovery steam generator which heats liquid supplied to an exhaust gas economizer using waste heat of exhaust gas supplied to the exhaust gas economizer; an organic rankin cycle generator which generates electricity by being operated by an organic refrigerator with a lower boiling point than water as a working fluid; and a heat transferring device which recovers the waste heat of heat exchange exhaust gas discharged after heat exchange with the liquid and a heat resource of scavenge air and transfers the waste heat and the heat resource to the organic rankin cycle generator.
