The effect of disposing the oil wastewater in Liaohe Oilfield by sunlight/Fenton method was studied in this paper, and the reason why the sunlight/Fenton method was better than the common Fenton method was analyzed. The wastewater mainly contains benzene and alkane compounds. Effect of sunlight/Fenton method on oil field wastewater treatment in Liaohe was related to height of the water layer. The removal rate of aqueous COD decreased when the height of the water layer increased, and the penetration ability of sunlight to this oilfield wastewater was about 1 m. When the height of wastewater was about 40 cm, COD of wastewater reduced from 430 to 63.2 mg·L-1 after treatment of oil wastewater by sunlight/Fenton method. It is proved that the form of the poly ferric compounds generated in the system by using the UV-Vis spectroscopy, infrared spectroscopy (FTIR) and X-ray diffraction method (XRD) is the same as the poly ferric compounds generated by the common Fenton system. At the same time, the transformation processes of benzene and alkanes were studied by UV-Vis spectroscopy which indicated that the transformation processes of alkanes in sunlight/Fenton were as same as the common Fenton method; enzene could be converted into photosensitive molecules activated by hydroxyl radical in sunlight/Fenton system. Thus, the sensitivities of the aromatic intermediates are the reason why the sunlight/Fenton method is more efficient than the common Fenton method.