A warning device for detecting water level in drainage well includes at least one level sensor and a transceiver. The level sensor includes an antenna and a first processing unit connected to the antenna. The first processing unit controls the antenna to transmit a signal to the transceiver as to increased fluid levels. The transceiver includes a first communication module and a second processing unit, the first communication module receives the signal and the second processing unit obtains the signal from the first communication module and determines the location of the level sensor and its urgency according to whether the signal decays or disappears because of inundation by drainage fluid. The second processing unit further sends information of the target level sensor to a terminal device.