The invention provides a nitrogen-doped graphene sheet and a preparation method and an application thereof. The nitrogen-doped graphene sheet is prepared in the presence of ammonia water serving as a nitrogen source by adopting a two-step reaction method. In the reaction process, no surfactant is used, the components in reactants are simple and the reaction conditions are mild, and therefore the problems of severe reaction condition, low nitrogen content, high production cost and the like in existing preparation method can be solved. In the preparation process, ammonia water is added to promote the reduction of graphene oxide to graphene. Meanwhile, ammonia water is used as a nitrogen source to prepare the nitrogen-doped graphene sheet. The invention can be used to prepare clean nitrogen-doped graphene sheet with high quality, and the nitrogen-doped graphene sheet has an excellent electrochemical performance and can be used for preparing an electrode material of a supercapacitor.