The invention discloses a preparation method of a narrow band-gap carbon nitride visible light photocatalyst, and belongs to the technical fields of material preparation and solar photocatalysis. The preparation method comprises the following steps: 1) dispersing cyanuric chloride and trithiocyanuric acid into an organic solvent according to the molar ratio of 1: (0.2-5), and stirring at the room temperature to obtain a reaction mixture; 2) transferring the reaction mixture into a tetrafluoroethylene-lined hydrothermal reaction still, and carrying out heat treatment to obtain the narrow band-gap graphite phase carbon nitride visible light photocatalyst. The invention also discloses the catalyst prepared through the preparation method. According to the preparation method, the band gap width of carbon nitride is decreased, the visual light absorption range is enlarged, and the photon-generated carrier separation and migration efficiency is improved; the catalyst prepared through the method is stable in property, nontoxic, and easy to recycle, can be circularly used, conforms to the requirement of practical production, and facilitates large-scale popularization.