The invention discloses quantitative test device and method for dissolving nitrogen dioxide in water. The quantitative test device mainly comprises a first step device and a second step device. The first step device comprises a large test tube and a small test tube of same height, the bottom of the small test tube is provided with small holes, the small test tube is nested in the large test tube and is disposed in a jar together with the same, concentrated nitric acid is fully filled in the small test tube and a copper piece is arranged in the small test tube. An orifice of the small test tube is plugged by a rubber plug, and concentrated nitric acid is also filled in an interlayer of the small test tube and the large test tube. The second step device is used for lifting the small test tube in the first step device from the large test tube, filling nitrogen dioxide gas into the small test tube and placing the small test tube as well as the large test tube into a wide-mouth container with water, the bottom end of the small test tube is slightly lower than the liquid level of the concentrated nitric acid of the large test tube, and the water level of the wide-mouth container is higher than the bottom end of the small test tube. By preparing the nitrogen dioxide by the first step device and quantitatively measuring the volume of the nitrogen dioxide dissolved in the water by the second step device according to the volume of the water entering the small test tube, impurity of collected gas by gas exhausting and collection method is avoided.