Abstract: A secondary water supply disinfection test was conducted by adding sodium hypochlorite in water samples in order to analyze the effects of pharmaceutical dosage, NH3-N, CODMn, pH and some other factors on the residual chlorine decay and chloroform and carbon tetrachloride formation in this research. The results of the test showed that: with the increased of pharmaceutical dosage, the amount of chloroform formation and generation rate increased gradually, but the lower yields of carbon tetrachloride and no significant variation. The generation of chloroform and carbon tetrachloride reduced with the increased of ammonia dosage. The increased of CODMn in raw water can promote the formation of chloroform and carbon tetrachloride. Within a certain range, as the pH increased, chloroform and carbon tetrachloride were increasing. And in the tested water samples，methylene chloride, trichloroethane, dibromo-chloro-methane, and other disinfection byproducts were not found.