In order to know the interaction effects between biofilm and chlorine decay, influences of chlorine residual on biofilm formation and chlorine decay rules under the existence of biofilm were investigated through static and dynamic simulation in simulated pipelines. Three kinds of pipe materials (cast iron, PE, stainless steel) and 3 chlorine dosage (1, 3, 4 mg/L) were chosen. Results show that chlorine decay and biofilm development interacted, and the specific results are as follows. With the increase of chlorine, biomass decreases in PE and stainless steel, while increases in cast iron; with the existence of biofilm, chlorine decay could be divided into two parts: fast decay (0-30 min) and stable decay (after 30 min). Cast iron influences chlorine decay seriously, followed by PE, and the effects of stainless steel is minimum. Influence of biological diversity on chlorine decay is not obvious, while biomass is closely related with chlorine decay. Velocity and chlorine dosage influenced chlorine decay evidently. This research could support disinfection, chlorine decay modeling and water quality security in water distribution network