The invention relates to a method for processing sewage, and concretely relates to a method for processing oil field sewage. The oil field sewage (purifying waste water) processed by the method can be used for preparing an oil extraction polymer solution. According to the method for processing oil field sewage, the oil field sewage passes through a forced ventilation opening spray tower for cooling, then passes through an A/O biochemistry system for biochemical treatment, Ca(ClO)2 is added for oxidation treatment, then filtering treatment is carried out through a multimedium filter filled with active carbon to obtain the purifying sewage, technology adaptability is strong, wide water temperature and water quality fluctuation range can be born, operation is simple, operation is stable, purifying sewage processed by the treatment process can reach the requirement for preparing the oil extraction polymer solution, the viscosity of the prepared oil extraction polymer solution is high, and viscosity stability is good.