To study the influences of dissolved oxygen(DO) on denitrification ability of biofilm-electrode,a system consisting of electrolysis reactor and biofilm-electrode reactor was fabricated.The effects of electrolysis current of electrolysis reactor system on the DO content in influents and the DO grads in biofilm-electrode reactor were investigated.Results implied that both microbe respiration and electrochemistry had effects on DO consumption in biofilm-electrode reactor system.The DO,the removal of nitrate-N and the nitrite-N in the biofilm-electrode system were measured when the current in the electrolysis reactor was applied in continuous and batch changes.Although the DO in cathodes area had a little increasing with the increasing of DO in influent,the DO values were all below 2 mg/L.No evidence proves the influences of DO on nitrate removal but the negative influences might be obtained on nitrite accumulation.Some changes on microbial species of bio-film electrode were found at different DO concentrations by transmission electron microscopy micrographs.