Abstract: The polymer technology pilot test for AY well pattern was initialed in November, 2005 in Bohai oilfield. The water cut of well pattern rose to 57.9% before polymer flooding. The increase of water cut was effectively controlled after polymer flooding trial. However, it encountered with some problems in the process of polymer injection, such as water cut of well A-1, A-4, A-2 increased by 10.0%~22.2% because water injection rate of external injection well B-14, A-12 and A-13 was enhanced which led water breaking through into well A-1, A-4, A-2. On the other hand, polymer from circumjacent well A-7, A-6 and other wells broke through to well AY. Water cut of well AY increased by 32% and polymer concentration increased by 150 mg/L. Measures such as transferring external water injection wells such as B-14, A-12 and A-13 to polymer injection wells and enhancing polymer concentration of central block from 1 750 mg/L to 2 500 mg/L were taken to decrease water cut of wells influenced by external water injection wells by 8%. On the other hand, water cut of the center well drop down by about 16% and output polymer concentration decreased by about 100 mg/L. Those measures effectively inhibited water breaking through and ensured good impact of polymer flooding development.