The invention provides a real-time sampling device suitable for being used in methane detection in a water drainage port during coal bed methane development. The device comprises a closed water tank,the top of the water tank is provided with a gas outlet, a water inlet is arranged at the upper part of the sidewall of the water tank, and a water outlet is arranged at the lower part of the sidewallof the water tank; a spirally downward water channel is arranged in the water tank, one side of the water channel is close to the inner wall and is in point-like connection with the inner wall, a silt for water to downward flow exists between the non-point-like connection position of the one side and the inner wall, a water baffle plate is arranged at the other side of the water channel to blockthe water from flowing out from the other side, and the starting position of the water tank communicates with the water inlet to introduce all gas and water mixture discharged from the water drainageport and directly discharged methane gas into the channel; and the water tank is also provided with a liquid level controller to keep the liquid level in the water tank at the plane of the water outlet during the detection. The sampling device guarantees the reliability, the accuracy and the real-time property of a detection result.