Provided is a drainage device and a system for controlling discharging of concentrated water in a water purification process. The device includes a valve cavity, a flowmeter, a transmission shaft and a control unit. The control unit is used for controlling a motion of the transmission shaft to adjust a liquid flow of the flowmeter in the valve cavity. The device inhibits an instantaneous impact of a liquid on the valve cavity to relieve vibration generated by the instantaneous impact; a special structural design is employed, which is favorable for controlling a flow of the concentrated water; moreover, the device is suitable for different pressure environments, achieves the objectives of small volume, controllable fitting area and small friction force; prevents leakage, and is easy to maintain and clean; and abnormal discharging of the concentrated water caused by water hammer and liquid impact in a water purification system is solved.