The invention discloses a device for measuring gas content of insulating oil. The device comprises an oil quantitative container, a vacuum degree measuring device and an oil pumping device, wherein an oil inlet is formed in a bottom end of the oil quantitative container; an oil outlet is formed in a top end of the oil quantitative container; the vacuum degree measuring device is connected with the oil quantitative container; valves are arranged between the water inlet, the oil outlet as well as the vacuum degree measuring device and the oil quantitative container; the oil pumping device comprises an oil pumping container and a piston arranged in the oil pumping container; an oil discharge outlet is formed in one end of the oil pumping container, and the same end is connected with the oil quantitative container; the other end of the oil pumping container is provided with an opening communicating with the atmospheric air; valves are arranged between the oil discharge outlet as well as the oil quantitative container and the oil pumping container; a side wall of the oil pumping container is provided with a plurality of volume markers. Meanwhile, the invention discloses a measuring method of the device. By adopting the device and the method, absolute vacuum of an initial state is realized through movement of the piston, and measurement of the gas content of the insulating oil is realized without pre-vacuuming, so that a test flow is simplified, and test time is shortened.