The invention belongs to the fields of energy and chemical industry and discloses a process for synthesizing LNG (Liquefied Natural Gas) and coproducing liquid ammonia by using methanol purge gas. The process comprises the steps: directly preheating the methanol purge gas generated from methanol synthesis to the temperature of 250 DEG C to 350 DEG C under the action of a methane synthesis catalyst, and enabling the preheated methanol purge gas to enter a methane synthesis unit for conducting a methane synthesis reaction; cooling the obtained methane gas and dehydrating the cooled methane gas, then, subjecting the dehydrated methane gas to a cryogenic separation liquefying procedure, obtaining the LNG from a tower bottom of a rectification tower, obtaining hydrogen gas, nitrogen gas and hydrogen-enriched gas which contains a small volume of methane from the tower top, recovering cold energy from the hydrogen-enriched gas, and then, subjecting the hydrogen-enriched gas to a pressure swing adsorption separating procedure, wherein the yield of the LNG is 95% to 100%; and separating the hydrogen gas, the nitrogen gas and the methane in a pressure swing adsorption separating device by the hydrogen-enriched gas so as to separately obtain the hydrogen gas, the nitrogen gas and the methane, then, pressurizing the nitrogen gas, mixing the pressurized nitrogen gas with the hydrogen gas, then, subjecting the gas mixture to an ammonia synthesis procedure in accordance with that the hydrogen/nitrogen ratio is 2.9 to 3.1, pressurizing the gas mixture to the pressure of 12.0MPa to 30.0MPa after the hydrogen gas and the nitrogen gas which meet a condition are mixed, and enabling the gas mixture to enter an ammonia synthesis tower so as to produce the liquid ammonia.