A preparation method of a graphene nanoribbon comprises the following steps: preparing a carbon nanowall; mixing the carbon nanowall with a metal intercalation agent in a mol ratio of 1-5:1 in an oxygen-free environment and heating the mixture to 200-1000 DEG C in a vacuum environment, maintaining the temperature, and carrying out reactions for 12-120 hours, so as to obtain a metal intercalation carbon nanowall, wherein the metal intercalation agent is at least one of lithium, sodium, potassium, magnesium, calcium, rubidium, strontium and barium; mixing the metal intercalation carbon nanowall with an ionic liquid in a mass volume ratio of 1 g:10-100 ml, treating the mixture with ultrasonic wave with the power of 2000-10000 W for 0.5-30 minutes, so as to obtain a reaction liquid; and filtering the reaction liquid to obtain the graphene nanoribbon. The graphene nanoribbon prepared by the preparation method has relatively high conductivity.