NOVELTY - Liquefying of sea natural gas comprises transporting pretreated natural gas into heat exchanging cooling box; introducing carbon dioxide into heat exchanging cooling box after constantly depressurizing and cooling; compressing pre-set nitrogen as high-pressure nitrogen; introducing it into cooling box for pre-cooling; flowing back portions of low pressure nitrogen to cooling box; pre-treating natural gas and pre-cooling by carbon dioxide gas; sending it into heavy hydrocarbon separation unit; and removing heavy hydrocarbon by gas-liquid separator and rectifying column.    USE - Liquefying of sea natural gas.    DETAILED DESCRIPTION - Liquefying of sea natural gas comprises:    (A) transporting natural gas mined from sea bottom to heat exchanging cooling box after pretreatment;    (B) introducing carbon dioxide gas pre-set in carbon dioxide pre-cooling circulation system to heat exchanging cooling box after being pressurized to 6 MPa and cooled to 20 degrees C until fully liquefied; further depressurizing liquid carbon dioxide to 0.6 MPa by throttling valve and cooled to - 52 degrees C as gas and liquid phases for pre-cooling input natural gas and high pressure nitrogen in heat exchange cooling box;    (C) compressing nitrogen pre-set in double-nitrogen expansion refrigerating circulation system at 8 MPa to changed it into high-pressure nitrogen; transporting high-pressure nitrogen to heat exchange cooling box; pre-cooling it by carbon dioxide to - 50 degrees C; dividing into two portions by flow divider with one portion being directly depressurized to 2 MPa and cooled to - 115 degrees C by expansion machine, and other portion being continuously pre-cooled to - 85 degrees C by low-pressure nitrogen flowing back to heat exchange cooling box, and expanded by nitrogen expansion machine, such that pressure is reduced to 1.4 MPa and temperature is reduced to - 156 degrees C; flowing back expanded portions of low pressure nitrogen to heat exchange cooling box to cool high-pressure nitrogen and natural gas, and to liquefies and supercools natural gas; subjecting portions of nitrogen that is heat exchanged and heated into multi-stage compression at 8 MPa using expansion machine pressurizer and multiple nitrogen compressors; cooling with water cooling machine; sending back natural gas to heat exchange cooling box and continuously pre-cooling by carbon dioxide to - 50 degrees C;and    (D) introducing pre-cooled natural gas into heavy hydrocarbon separation unit and slightly cooling by back-flowing nitrogen; removing heavy hydrocarbon by gas-liquid separator and rectifying column; cooling to - 151 degrees C to make it fully liquefied and supercooled, and throttled and reduced temperature to - 159 degrees C by throttling valve, so that 7% of liquid is gasified; introducing gas-liquid mixture to liquefied natural gas buffer tank; sending liquid phase to storage tank for storage; and discharging and collecting gas phase from buffer tank top.    An INDEPENDENT CLAIM is included for liquefying device comprising heat exchange cooling boxes, carbon dioxide pre-cooling circulation system, double-nitrogen expansion refrigerating system, and natural gas heavy hydrocarbon removing and liquefying system. The carbon dioxide pre-cooling circulation system comprises carbon dioxide compressor, water cooling machine and carbon dioxide throttling valve connected to heat exchange cooling box. The double-nitrogen expansion refrigerating system comprises nitrogen flow divider connected with heat exchange cooling box; and pipeline connected with nitrogen expansion machine, which is connected to expansion machine pressurizer. Inputs of pipelines are connected to heat exchange cooling boxes, and that their outputs are successively connected with water cooling machine, nitrogen compressor, mixing machine, and heat exchange cooling boxes. The natural gas heavy hydrocarbon removing and liquefying system comprises primary separator with output being divided into two pipelines, such that one pipeline is directly connected to rectifying column middle part and other one is connected with rectifying column top part. The top part of rectifying column and secondary separator are connected to other heat exchange cooling boxes, natural gas throttling valve and liquefied natural gas (LNG) buffer tank through mixing machine. The output of LNG buffer tank is connected with LNG storage tank. The bottom part of rectifying column is connected with throttling valve, water cooling machine and LNG buffer tank. The output of LNG buffer tank is connected with LNG storage tank.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of liquefying device for liquefying sea natural gas.