The invention discloses a method and a device for metering oil in an oil-water separator. The device comprises at least two measuring cylinders which are sequentially arranged, wherein openings are formed in the upper ends of the measuring cylinders; bases are arranged at the lower ends of the measuring cylinders; bottom water outlets are formed in the bottom side walls of the measuring cylinders; a diversion branch pipe is arranged between every two adjacent measuring cylinders; the inlet end of the diversion branch pipe is communicated with the bottom water outlet of the previous measuring cylinder; the outlet end of the diversion branch pipe extends upwards to the upper end opening of a subsequent adjacent measuring cylinder; and the height of the culminating point of the diversion branch pipe is smaller than that of the upper end opening of the measuring cylinder connected with the diversion branch pipe. According to the method and the device disclosed by the invention, the separation is carried out by using the oil-water density difference principle; due to the adoption of a plurality of measuring cylinders which are arranged in parallel and communicated in one way, full oil-water separation of oil-water mixtures generated by an indoor oil displacement experiment is realized; and the method and the device have the advantages of high accuracy, simple structure, low cost and convenience in cleaning and are suitable for carrying out oil-water separation on the oil-water mixtures generated by the indoor oil displacement experiment by oil extraction for three times.