Disclosed is a reverse-flow air floating settling water-purifying tank, which is used for oil field sewage treatment. The water-purifying tank is characterized in that: an air-floating purifying separating system and a gravity settling purifying separating system are distributed to form a ring-shaped concentric circle, the ring-shaped space between a central pillar and a central cylinder can be used for the air floating purifying separation, and the ring-shaped space between the central cylinder and a steel tank can be used for the settling purifying separation; the water from the oil field firstly enters into a reverse-flow air floating purifying separating system which is composed of the central cylinder, a hydraulic dregs-collecting cone, a dregs-collecting device, a ring-shaped water-distributing device, a communicating pipe, a ring-shaped air-dissolved water releasing pipe and a ring-shaped water-collecting pipe; the purified oily sewage after air floating purification can be treated through the gravity purifying separation by the gravity purifying system which is composed of the gravity water-distributed system, a gravity water-collecting system and an oil-collecting groove, so as to improve the unit volume treating density and the space utilization ratio of the vertical type oil-removing tank, and improve the treating effect and the capability. Proved by experiment, the water-treating capability can be improved by over 3 times, and the oil and the suspended substance contained in the outlet can be respectively reduced by over 80% and 60%.