The embodiment of the invention provides a method for sampling oil. The method is based on an oil sampling device which comprises a disc which is arranged on an oil outlet of a sampling valve of a transformer, a first tubular channel which is connected with the disc and comprises a first channel and a second channel, and a second tubular channel which is connected with the first tubular channel and comprises a third channel and a ball valve; the method comprises the steps that the rotary ball valve drives a push rod which is connected with the ball valve to extend; the push rod jacks a steel ball at an oil outlet channel opening of the second channel; an oil sample flows into an oil cavity which is arranged at the connection part of the third channel and the second channel after the steel ball is jacked open; the oil sample in the oil cavity flows into a central channel of the push rod through a through hole which is formed in the surface of the push rod; the oil sample flows into the ball valve which is connected with the push rod through the central channel of the push rod; and after an oil sampling opening of the ball valve is opened, the oil sample is taken out. By adopting the method of the embodiment, oil is conveniently sampled.