The invention relates to a preparation method of a carbon dioxide reduction visible light response ultrathin Bi4O5Br2 photocatalyst. Compounds containing 5.1 mmol of bismuth elements are dissolved into a certain amount of propanetriol; then, 2 mmol compounds containing bromine elements (Bi:Br=2.55:1) are dissolved into a certain amount of propanetriol; the propanetriol solution of the compounds containing the bromine element is dripped into a propanetriol solution of bismuth nitrate pentahydrate; stirring reaction is performed for 60min; the mixed solution is transferred into a reaction kettlefor reaction for 16h under the condition of 160 DEG C; cleaning and drying are performed to obtain reaction precursors; 0.4g of precursors are weighed and are added into 300mL of distilled water; hydrolysis is performed at the water bath temperature being 50 DEG C for 24h; obtained solid powder is dried to obtain the ultrathin Bi4O5Br2 photocatalyst. The prepared catalyst has the enhanced response to the visible light, has higher catalysis activity, particularly has high activity on carbon dioxide reaction, and can be applied to carbon dioxide reaction removal.