Mercury possesses the characteristics of hypertoxicity,atmospheric migration and bioaccumulation,which has posed a serious threat to the ecological environment and human health. The mercury emission in China from coal combustion ranks first in the world,therefore,it is imperative to strengthen the research on mercury control technology. It is the key to control mercury pollution by the efficient oxidation of elemental mercury (Hg0) from coal-fired flue gas. Using Ag/AgCl modified Bi5O7I/Bi2O3 composite as photocatalyst,the effects of experimental parameters such as Ag/Bi mass ratio,pH,inorganic anion,SO2 and NO on the removal of gaseous elemental mercury by photocatalyst were studied in a wet photocatalytic reactor. The photocatalysts were characterized by N2 adsorption-desorption,XRD,SEM,DRS,XPS and ESR. The results show that Ag/AgCl modified Bi5O7I/Bi2O3 composite possesses the superior photocatalytic oxidation performance and the mercury removal efficiency can reach as high as 98.5% when the Ag/Bi mass ratio is 0.03. With the increase of pH and the addition of inorganic carbonate ions,the efficiency of mercury removal decreases significantly. Compared with NO,SO2 has a greater inhibitory effect on gaseous Hg0 removal. The chemical species of Bi and O in Bi5O7I/Bi2O3 will be varied due to the addition of Ag/AgCl,and a Z-scheme heterojunction structure is formed between Ag/AgCl and Bi5O7I/Bi2O3,which plays a crucial role in the activity of quaternary composite photocatalytic materials. The experiments of free radical capture show that superoxide radical (·O-2) and holes (h+) were the main active substances for Hg0 removal.