Abstract: At room temperature，the photocatalyst of Bi12O17Cl2 was synthesized with bismuth trichloride pentahydrate, sodium hydroxide and ethanol as raw materials.Bi12O17Cl2/TiO2 of composite photocatalyst was prepared to composite TiO2 and Bi12O17Cl2 by mechanical mixing method.By changing the mass percentage content of Bi12O17Cl2 in Bi12O17Cl2/TiO2, the photocatalytic degradation efficiencies of Bi12O17Cl2/TiO2 ,TiO2 under ultraviolet and visible light conditions were investigated，respectively.By researching and finding, the UV photocatalytic activity of 3% Bi12O17Cl2/TiO2 was the highest when UV light illuminated for 1.25h, and the photocatalytic degradation efficiency of methyl orange was 100% with 3% Bi12O17Cl2/TiO2 as catalyst.When visible light illuminated for 2h, the visible light catalytic activity of 3% Bi12O17Cl2/TiO2 was the highest, and the photocatalytic degradation rate of methyl orange was 90% with 3%Bi12O17Cl2/TiO2 as catalyst.When visible light illuminated for 2 h, the COD removal rate of photocatalytic degradation of methyl orange solution was the highest by 3% Bi12O17Cl2/TiO2, and its COD removal rate was 75%.The study on the photocatalytic reaction mechanism of Bi12O17Cl2/TiO2 shows that titanium dioxide has a more negative conduction value than Bi12O17Cl2, and TiO2 also has a more corrected valence band value than Bi12O17Cl2.Under light conditions, electrons locating in the conduct band of titanium dioxide can migrate to the conduct band of Bi12O17Cl2 . At the same time, the holes locating in the valence band of titanium dioxide can migrate to the valence band of Bi12O17Cl2, so that the recombination rate of photogenerated electrons and holes in TiO2 can be effectively reduced, and the photocatalytic degradation rate of Bi12O17Cl2/TiO2 can be improved.