The utility model mainly discloses a system ammonia synthesis gas coproduction LNG's device is washed with liquid nitrogen adopts the liquid nitrogen to wash with rectifying column technology, retrieves methane and arranges ammonia synthesis technology anterior segment in, and the methane rate of recovery can reach more than 99%, and hydrogen recovery is rateed and can be reached more than 99.5%, has effectually reduced the influence of inert gas to the synthetic ammonia process, disclose the nitrogen inflation simultaneously and added precooling machine refrigeration technology, reduced the area of device, disclose the cold energy utilization of supporting air separation device liquid nitrogen product simultaneously, reduced the refrigerator energy consumption, improved the economic nature of device.