The invention relates to a ship cooling system and cooling method for composite using of LNG and refrigerating unit cold energy. A cold media outlet of a cooling coil is connected with a first three-way valve which is connected with a cold media inlet of a refrigerating unit, a second three-way valve and an LNG heat exchanger respectively; a cold media outlet of the refrigerating unit is connected with the second three-way valve; the second three-way valve is connected with a third three-way valve and a cold media inlet of cooling coil respectively; the third three-way valve is connected with a cold media inlet of the cooling coil and a cold media outlet of the LNG heat exchanger respectively; LNG in an LNG tank absorbs heat through the LNG heat exchanger and is gasified through an LNG gasifier, and finally, gas fuel is provided for an LNG fuel engine. According to the ship cooling system and cooling method for composite using of the LNG and the refrigerating unit cold energy, while cold quantity is provided by LNG gasification heat absorption, ship cold quantity supply can be adjusted freely according to different conditions such as load of the LNG fuel engine and ship cold quantity requirement, so that the energy saving purpose can be realized, and the problem of cold quantity waste or insufficient cold quantity supply can be solved.