Nitrogen methane single-expansion natural gas liquefaction system with pre-cooling function

The utility model provides a nitrogen methane single-expansion natural gas liquefaction system with pre-cooling function. The system comprises mutually-connected nitrogen methane expansion refrigerating cycle systems, and a natural gas de-heavy-hydrocarbon and liquefaction system, wherein the natural gas de-heavy-hydrocarbon and liquefaction system comprises a pre-cooling heat exchanger, a pre-cooling machine unit, a main heat exchanger, a heavy hydrocarbon separator and a heavy hydrocarbon storage tank both connected onto the pre-cooling heat exchanger, as well as an LNG (Liquefied Natural Gas) flash vaporization tank and an LNG storage tank both connected onto the main heat exchanger; the nitrogen methane expansion refrigerating cycle systems comprise expansion machines, booster compressors and nitrogen methane compressors , which are coaxially driven and sequentially connected; and booster compressor aftercoolers are connected onto the booster compressors. The system provided by the utility model has the b