The invention provides a preparation apparatus for a standard transformer oil sample, comprising a gas tank, an oil cup and an enclosed cylinder body, wherein the outlet of the enclosed cylinder body is connected with the inlet of the enclosed cylinder body via an oil outlet valve, a circulating pump and an oil inlet valve so as to form a circulation loop; the oil cup is connected with the gas tank via a three-way valve and connected with the circulation loop via a liquid flow pump; the other end of the oil cup is provided with an exhaust valve and a thermometer; the enclosed cylinder body is provided with a vacuum-pumping port, a gas source port, a sampling port, an oil inlet and an oil outlet; and the enclosed cylinder body controls full mixing of transformer oil samples via the circulating pump so as to obtain the standard transformer oil sample. Compared with the prior art, the preparation apparatus for the standard transformer oil sample in the invention has the following advantages: the prepared standard transformer oil sample can be used for comprehensive evaluation of on-line chromatographic performance including data accuracy, minimal detection concentration, the calibration mode of the apparatus and the like; and on-line detection apparatuses in operation can be checked on site and defects of the on-line detection apparatuses can be eliminated according to data.