The invention relates to an oil separating device for treating small-scale oil-containing waste water, in particular to a small automatic multi-stage oil-containing waste water treating device, which consists of a water inlet pipe, a fine screen, a granule removing area, a gravity oil-removing area, an oil collecting pipe, an absorption area, a PLC water level automatic control device, an oil-separating felt drawer, an activated-charcoal column and a drain pipe, wherein the water inlet pipe is arranged at the top end of the granule removing area; the fine screen is arranged between the granule removing area and the water inlet pipe and is slightly higher than the lowest water level; the granule removing area is communicated with the gravity oil-removing area; the oil collecting pipe and the PLC water level automatic control device are arranged at the upper end of the gravity oil-removing area, and the oil collecting pipe is as high as or slightly higher than the highest water level; the gravity oil-removing area is communicated with the absorption area through a gate which is arranged at the bottom of the gravity oil-removing area; the absorption area consists of the oil-separating felt drawer and the activated-charcoal column which are sequentially arranged from the bottom to the top and sequentially communicated with each other, and the upper end of the activated-charcoal column is slightly higher than the lowest water level; and the drain pipe is arranged at the bottom of the absorption area. The oil in the oil-containing waste water is effectively removed by the device. The treated water can enter a drain pipe net and also can be directly discharged.