Study on formation process of biological purification efficiency in raw water delivery pipelines was beneficial to improve water quality in long distance raw water delivery pipes. Using simulated pipeline reactors to simulate water situation in raw water delivery pipelines, and water samples were prepared in laboratory. The influent and effluent water quality, the microorganisms in the biofilm is continuously monitored. Results show that the ammonia nitrogen (NH₄+-N) and organic matter removal are stabilized after a gradual increase in first, and stable value decreases slightly compared to the maximum in the formation process of biological purification efficiency. Biofilm grow on polyethylene slides in simulated pipeline reactor and then harvest within 45 days, the removal rate of NH₄+-N remains stable at 75％. Nitrite nitrogen (NO₂--N) effluent concentration remains stable at 0.02 mg／L. Ammonia oxidizing bacteria (AOB) is dominant strain compared to nitrite oxidizing bacteria (NOB) in raw water delivery pipes. Removal rate of UV₂₅₄ and turbidity are about 30％, and orthophosphate (PO₄3-) removal rate ranges from 13.64％ to 38.78％.