The invention relates to the technical field of transformer oil analysis and inspection, in particular to a device for on-site inspection of an online monitor for dissolved gas in transformer oil. The device comprises a gas bottle and oil sample tanks, the gas bottle is connected with a three-way valve through a pipeline, the three-way valve is connected to the oil sample tanks and an oil collection tank through a gas feeding pipe and an oil feeding pipe respectively, the oil sample tanks include multiple testing oil sample tanks and a new oil tank, an elastic oil-resistant airbag is arranged inside each oil sample tank, an oil sample is contained in each elastic oil-resistant airbag, a bottom outlet of each elastic oil-resistant airbag is communicated with the oil feeding pipe, a collection valve is arranged on the oil feeding pipe correspondingly to the oil collection tank which is connected with a vacuumizing device, and an inspection branched pipe is arranged on the oil feeding pipe and connected to a to-be-inspected device through an inspection valve. The device is simple and portable in structure, suitable for on-site inspection of the online monitor for the dissolved gas in the oil and high in inspection accuracy. The invention further provides a method using the device. The online monitor for the dissolved gas in the oil can be inspected quickly, and inspection accuracy is guaranteed.