The embodiment of the invention discloses a water channel flow velocity and flow rate measuring method, device and system. The system comprises an unmanned aerial vehicle, a processor which carries out data transmission, and a radar chronograph. The processor automatically plans a flight course of the unmanned aerial vehicle according to position information of each measuring point, measuring duration and a preset flying height so that the radar chronograph installed on the unmanned aerial vehicle can measure the flow velocity of each measuring point of a water channel to be tested during a flight process according to the flight course, and measurement data is sent to the processor through the unmanned aerial vehicle. The processor calculates and acquires the flow rate of the water channel to be tested according to the measurement data of the radar chronograph, the cross-section information of the water channel to be tested, water level data information, each preset flow measurement parameter value, and the position information of each measuring point. In the invention, water channel flow measurement operation can be remotely performed, a potential safety hazard of water channel flow velocity and flow rate measurement is eliminated, the personal safety of hydrological test personnel is ensured, and the efficiency and the accuracy of the water channel flow velocity and flow rate measurement are increased, which is good for improving hydrologic monitoring quality.