To improve the safety of distributing raw water, a BAR(Biofilm Annular Reactor)was constructed to simulate the distribution system and the effect of dissolved oxygen (DO) concentration on the effluent water quality and biofilm was studied. The 454-pyrosequencing technology was employed to analyze the diversity of biofilm in the reactor. Experimental results showed that the turbidity and the concentration of total iron, ammonia nitrogen decreased obviously with DO concentration increasing, while the concentrations of total nitrogen and COD_Mn changed slightly. The numbers of iron bacteria and sulfate-reducing bacteria reduced significantly, while the richness and diversity of the biofilm related bacteria (such as nitrifying bacteria) improved. So, increasing DO concentration can alleviate the pipeline corrosion and develop biofilm purifying water role.