The invention belongs to the technical field of flexible conductive fibers for wearable electronic devices and preparation thereof, specifically relates to a conductive coaxial nanofiber with a sheath-core structure of an aramid fiber or a polyamide fiber coating a reduced graphene oxide and a preparation method thereof, and provides a preparation method of a conductive fiber with a sheath-core structure. The preparation method of the conductive fiber with the sheath-core structure comprises the steps that (1), a core layer spinning solution is prepared; (2) a sheath layer spinning solution isprepared; (3) a coaxial fiber is prepared; and (4), high-temperature thermal reduction is conducted, specifically, the coaxial fiber obtained in the step (3) is subjected to the high-temperature thermal reduction to obtain a conductive coaxial nanofiber, that is, the conductive fiber with the sheath-core structure. According to the preparation method of the conductive fiber with the sheath-core structure, graphene oxide is used as the core layer material, and the high temperature resistant wet spinning polymer is used as the sheath layer material, thus reduction of the core layer graphene oxide through the high temperature thermal reduction can be realized, so that the obtained fiber has flexibility and also has the excellent electrical and mechanical properties.