The invention discloses a nano-cobalt/nitrogen-doped graphene composite material and a preparation method thereof. The method includes: adding the graphene oxide into ultrapure water for ultrasonic dispersion; adding dimethylimidazole into a methanol-ethanol system, and stirring to dissolve dimethylimidazole; mixing and uniformly stirring the two systems, adding a methanol-ethanol solution of cobalt nitrate, continuously and uniformly stirring the mixture, centrifuging and washing the mixture, collecting and drying the material, and heating the obtained material to 550 DEG C at the increase rate of 2 DEG C/min under the condition of argon and hydrogen mixed gas, and keeping the temperature for 8 hours to obtain the nano-cobalt/nitrogen-doped graphene composite material. The nano-cobalt/nitrogen-doped graphene composite material prepared by taking graphene oxide as a substrate has excellent catalytic performance on oxygen reduction reaction, the overpotential of the nano-cobalt/nitrogen-doped graphene composite material reaches 300 mV, and the nano-cobalt/nitrogen-doped graphene composite material is expected to be applied to electro-catalytic water decomposition and ionic membraneexchange batteries.