The invention discloses a preparation method of a photocatalyst based on monodisperse elemental bismuth and carbon nitride. The method comprises the following key steps: monodisperse elemental bismuth particles which are prepared through a liquid phase chemical reduction technology are immobilized on the surface of carbon nitride used as a carrier and prepared by using urea as a precursor, and a surfactant component in the above obtained composite system is removed through a coordination exchange reaction to obtain the elemental bismuth/carbon nitride photocatalyst. The preparation method of the elemental bismuth/carbon nitride photocatalyst has the advantages of simple process, low energy consumption, controllable material morphology and high reaction activity. The elemental bismuth/carbon nitride photocatalyst prepared in the invention can rapidly and effectively remove carbon monoxide and other pollutants in air, and the catalysis activity can be regulated through the controllable synthesis of the elemental bismuth.