The invention discloses a triple co-generation system for heating cargo oil and ballast water and performing refrigeration by waste heat of an oil tanker. The system comprises a waste heat boiler, a hot water pump, a generator, an expander, a compressor, a cargo oil hating coil, a condenser, an evaporator, a working medium pump, a cooling water pump, a ballast water heating coil, a first valve, a second valve, a third valve, a fourth valve, a fifth valve and a connecting pipeline, wherein the outlet of the generator is connected with the first valve and the third valve which are used for enabling the cargo oil heating coil or the expander to work respectively, the generator forms a loop together with the cargo oil heating coil through the first valve, and the outlet of the cargo oil heating coil is connected to the inlet of the generator; the outlet of the generator forms an organic Rankine cycle together with the expander, the condenser and the working medium pump through the third valve; the compressor forms a refrigeration cycle together with the condenser, the fourth valve and the evaporator. According to the system, the three functions of heating cargo oil and ballast water and performing refrigeration are realized by recovering the waste heat, and the energy-saving effect is remarkable.