The invention belongs to the technical field of chemical engineering and specifically discloses a novel process of preparing liquefied natural gas. The process comprises the steps as follows: purifying and pre-purifying a CO2-rich gas to obtain the CO2 gas with content over 95%, mixing the gas with a hydrogen-rich gas, carrying out deep hydrodesulfurization to obtain a gas mixture rich in CO2 and H2 with total sulfur less than 0.05ppm; pre-treating, compressing and purifying a cove-oven gas, carrying out multi-stage external-circulation heat-insulation methanation with the gas mixture rich in CO2 and H2, carrying out molecular sieve deep dehydration, and finally carrying out copious-cooling, separating and liquefying to obtain the liquefied natural gas, the hydrogen-rich gas and a nitrogen-rich gas, wherein the hydrogen-rich gas is returned to an inlet of a material gas compressor for being recycled, and the hydrogen-rich gas is applied as a tail gas for being returned to a coking unit and used as a fuel. In the process, utilization rate of CO, CO2 and H2 is high, so that yield of the liquefied natural gas is improved, and product unit consumption is low; and meanwhile, a waste gas can be utilized to purify and sufficiently utilize carbon dioxide.