There is disclosed a device for purifying oily wastewater, including a vertical tank of an atmospheric pressure, inside which a central coagulation reaction tube, a reverse cone-shaped cyclone flocculation stage, a suspended sludge filtering zone and a purified water commutation stage of inclined plates are included from bottom to top sequentially, oily wastewater is sequentially subjected to a coagulation reaction, suspended sludge filtering and a purified water commutation process and then discharged, a sludge collecting groove collects and guides the suspended sludge to a sludge concentrating zone for further sedimentation and concentration processes and then discharges it to external to the tank, in this way, the purification reaction, flocculation and fine filtration of wastewater and the sludge concentration process are integrated within the same wastewater purifying device. Thus, the device is equivalent to the existing wastewater pretreatment system plus a fine filtration system.