LNG transport ship BOG treatment system and method using nitrogen and helium expansion process

The invention relates to an LNG transport ship BOG treatment system and method using a nitrogen and helium expansion process. The system comprises the following components: an LNG storage cabin and a cold box, wherein a liquid phase outlet of the LNG storage cabin is connected with a first inlet of the cold box, and a first outlet of the cold box is connected with a spraying pipeline at the top of the LNG storage cabin; and an expansion machine, a compressor and a cooler, wherein an inlet of the expansion machine is connected with a second outlet of the cold box, an outlet of the expansion machine is connected with a third inlet of the cold box, a third outlet of the cold box is connected with a fourth inlet of the cold box, a fourth outlet of the cold box is connected with an inlet of the compressor, an outlet of the compressor is connected with an inlet of the cooler, and an outlet of the cooler is connected with a second inlet of the cold box. The treatment system preferably further comprises an LNG immers