The invention discloses a composite visible light catalyst of a g-C3N4 / bismuth-based oxide and a preparation method and an application of the composite visible light catalyst. Carbon nitride in the composite light catalyst is graphite-phase carbon nitride; and the bismuth-based oxide is more than one of Bi2O2CO3, BiOBr, BiOCl, Bi2WO6, BiVO4 and BiOI. The components of the obtained composite catalyst are tight in contact, so that separation of photo electron-hole pairs is facilitated, and the photocatalytic efficiency is also improved; the composite catalyst has good degradation efficiency on organic pollutants represented by methyl orange under sunlight illumination; the preparation method disclosed by the invention has the characteristics of being simple in process, simple and convenient to operate, and cheap in raw materials; the prepared composite catalyst can be widely applied to waste water treatment, atmospheric purification and the like; and a new concept is provided for design of the novel composite visible light catalyst by the preparation method.