This paper mainly study the mechanism of removing COD of oilfield wastewater by oxidation, coagulation and oxidation-coagulation. The results show that with the increase of oxidant 1# dosage, the total content of the organic matter in wastewater reduces with removal rate of organic matter significantly correlated with the organic molecular structure. The highest removal rate of COD is 53.38% by single oxidation, and the oxidation can remove the pentacyclic organic matter (100%), the bicyclic organic matter (90.6%), tricyclic organic matter (49.48%), and tetracycline organic matter (38.45%) in wastewater. With the increase of coagulant 1# dosage, the total aromatic organic matter content decreased in wastewater with the removal rate of organic matter significantly correlated with the organic molecular structure and solubility (polar). The highest removal rate of wastewater COD is 42.78% by single coagulation, which can remove tricyclic organic matter (82.74%), the pentacyclic compounds (78.64%), tetracycline organic matter (74.93%), and the organic matter (73.35%) in wastewater. The highest removal rate of wastewater COD is 87.82% by oxidation-coagulation, which can remove the pentacyclic organic matter (100%); the tetracyclic matter(93.89%), tricyclic organic matter (91.87%), and bicyclic organic matter (77.69%) in wastewater. The results show that coagulation-oxidation can improve processing efficiency in organic wastewater, the mechanism of which may be interaction of substituted, oxidation, coagulation and stability, adsorption and precipitation, etc.