NOVELTY - A biological treatment agent for oil field sewage comprises following raw material components: 0.1 wt.% to 5 wt.% denitrifying-nitrifying bacteria solution, 0.05 wt.% to 0.5 wt.% of nitrate, 0.05 wt.% to 0.5 wt.% of salt, 0.001 wt.% to 0.01 wt.% double-quaternary ammonium salt type mud stripping agent, 5x 107 wt.% to 1x 106 wt.% of potassium permanganate, 1x 105 wt.% to 2x 105 wt.% source of potassium dichromate and the rest is denitrifying bacteria and nitrifying bacterial microorganism for oil field sewage.    USE - As biological treatment agent for oil field sewage (claimed).    ADVANTAGE - The agent provides indigenous richness and utility of beneficial microorganism, completely inhibits activity and growth of sulfate reducing bacteria, produces hydrogen sulfide, inhibits growth and corrosion of sulfate reducing bacteria caused to environment, avoids corrosion of oil field and reduces cost factors.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) preparation of the agent involving adding denitrifying-nitrifying bacteria liquid in oil field sewage of indigenous denitrifying bacteria and nitrifying bacterial microorganism culture, where denitrifying-nitrifying bacteria liquid is formed by adding 0.1 parts to 0.5 parts of nitrate, 0.1 parts of nitrite, 0.5 parts of salt, 0.001 parts to 0.01 parts of double quaternary ammonium salt type mud stripping agent and 98.99 parts to 99.8 parts of oil field sewage, continuing adding till denitrifying bacteria and nitrifying bacterial microorganism is fully mixed in a sealed container having pH set at 7 to 8, conducting mixing at low condition of temperature of 25 degrees C to 35 degrees C for 3 days to 15 days to obtain denitrifying-nitrifying bacterial liquid; and    (2) use of biological processing agent in oil field involving injecting indigenous denitrifying bacterial microorganism oil field sewage and nitrifying bacterial microorganism in oil field liquid, biologically processing, selecting content of sulfate reducing bacteria in the oilfield sewage to less than or equal to 106 per ml and injecting the treatment agent.