The invention provides a preparation method of a bismuth/bismuth oxide/carbon composite material by utilizing organic-inorganic complexation and calcination. In a degradation process of 2,4-dichlorophenol, a novel composite photocatalyst presents higher photocatalytic activity than pure bismuth oxide under same sunlight and visible light irradiation simulation conditions. The improved photocatalytic activity attributes to smaller particle size and more active sites. What is more important, a surface plasma resonance effect of bismuth can improve light absorption and promote photogenerated charge separation. In addition, carbon in a sample can improve light absorption and promote photogenerated carrier migration. Therefore, a photocatalytic property of the material can be improved. What is more important, the preparation method provides a novel clue for preparing a small-particle-size metal/metal oxide/carbon composite material.