The present disclosure provides an adjustable drainage structure. The adjustable drainage structure is disposed on a tank. The tank accommodates a cultivation fluid. The tank includes a bottom part. The bottom part has an opening. The adjustable drainage structure includes an adjustable tube and a fixing device. The fixing device is disposed on the opening. The fixing device has an inserting port communicating inside and outside the tank. The adjustable tube is disposed on the fixing device and extrudes away the bottom part. The adjustable tube has an intake hole opposite to a draining hole. The draining hole is communicated to the inserting port. The adjustable tube is opposite to the opening elongated or shortened.
