The invention discloses a device for extracting dissolved gas from soil water. The device mainly comprises a sample bottle, a gas sample tube, a cold trap, a vacuum gauge and a vacuum pump, wherein a balloon is arranged in the sample bottle and is communicated with the atmosphere outside the sample bottle through a duct; the duct outside the sample bottle is provided with a piston A; the gas sample tube is communicated with the sample bottle and the cold trap simultaneously through a piston B; a piston C and a piston D are respectively arranged between the sample bottle and the piston B, and between the piston B and the cold trap; the vacuum gauge connected with a vacuometer is arranged between the piston D and the cold trap; the cold trap is communicated with the vacuum pump through a piston E; and a temperature control cup and a liquid nitrogen cup are sleeved outside the sample bottle and the cold trap. The device can make the dissolved gas in the soil water or in a liquid phase ofa solid-liquid mixed phase completely released, prevents gas adsorbed by soil and atmosphere from polluting the dissolved gas, and effectively extracts the dissolved gas from the soil water.