The invention relates to a preparation method of a heterojunction with special exposed surface, and belongs to the technical field of preparation methods of photocatalytic materials. Semiconductor Co3O4 with an adjustable band gap is selected and is used for constructing a Co3O4/2D g-C3N4 heterojunction photocatalyst with the special exposed crystal surface together with 2D g-C3N4 with a liquid nitrogen assisted thermal oxidation method, the light absorption range is enlarged, and the active site density is increased, so that the capacity of photocatalysis of CO2 for reduction can be improved.Facts and results prove that the heterojunction can greatly improve photocatalytic performance and enhance CO2 adsorption and conversion.