Photoreduction of Hg (II) is of great importance but remains not fully understood especially in the presence of SO42-. In this study, laboratory experiments were conducted to investigate the reduction of Hg (Cl)2with various SO42-concentrations under artificial solar radiation. The whole process was tracked by changing Hg (0) concentrations in argon; the rate constants were calculated by trial method and were compared with other experiment. The results show the reaction rate decreased with increasing SO42-concentrations (0-20 mg L-1) and the cause of inhibitory effect is assumed with two explanations. The concentration of Hg (0) in argon increased firstly and decreased later in each treatment, since the main reactions in rising and dropping period are different. The comparison indicates that reduction rate is influenced by combined factors such as the form of mercury, the quantity of DOM and TSSs, depth of water and quality of light source.