The invention relates to N-TiO2/C and N-TiO2 and a preparation method thereof, and particularly discloses a method for preparing nitrogen-doped titanium oxide and a nano-composite of nitrogen-doped titanium oxide with carbon through heat treatment in air. The preparation method comprises the following steps: (1) mixing 10wt% of a tetramethylammonium hydroxide water solution with ethylene glycol, adding isopropyl titanate, uniformly stirring, and performing hydrothermal treatment on an obtained solution for 8 hours at 200 DEG C; (2) centrifuging and cleaning a hydrothermal product which is obtained in the step (1) in ethanol and deionized water, and drying in the air at 70 DEG C; and (3) performing heat treatment on a dried product which is obtained in the step (2) for 2 hours in the air at 250 to 550 DEG C, so as to obtain the nano-composite of nitrogen-doped titanium oxide/carbon or nitrogen-doped titanium oxide. The nano-composite of the nitrogen-doped titanium oxide/carbon and nitrogen-doped titanium oxide, which are obtained by using the preparation method, have the advantage of high nitrogen content and have visible light catalytic activity, and an absorption spectrum is expanded to a visible light region. The adopted preparation method has the advantages of simplicity, convenience, easiness in implementation, high safety factor, low cost and the like.