The invention relates to an alarm system, in particular to an alarm system for an ultrahigh water level in an urban river caused by heavy rain. The objective of the invention is to provide an alarm system for an ultrahigh water level in an urban river caused by heavy rain, which can monitor the water level all the time and can remind people in time. The technical scheme is that the alarm system for an ultrahigh water level in an urban river caused by heavy rain comprises a signal station, a riser, a fixed seat, a fixing screw, an L-shaped plate, a first acoustic Doppler flow velocity meter andthe like; the signal station and four fixed seats are arranged on the ground; and the signal station is located between the four fixed seats. The alarm system can detect the flow velocity and the water level of water in a river through the first acoustic Doppler flow velocity meter, a second acoustic Doppler flow velocity meter and a third acoustic Doppler flow velocity meter, and can ensure theaccuracy of detected data. Therefore, the water level in the river can be constantly monitored, and the detected data can be uniformly displayed on a background server, so that workers can promptly remind people that the water level in the river is too high.