A device for constructing an underground reservoir by dissolving limestone using carbon dioxide. The device includes a CO2 storage tank; an absorption tower; a decompression valve; a gas-liquid separator; a crystallizer; a vacuum pump; a buffer tank; a first booster pump; a second booster pump; and a third booster pump. The decompression valve is connected to a limestone layer, and is connected to the gas-liquid separator. The absorption tower is connected between the gas-liquid separator and the limestone layer.