During the flood period, the measurement using traditional contact hydrological monitoring equipment is highly dangerous, and sediment and floating objects easily interfere with the measuring equipment. This study explores the application of the image method for measuring flow velocity technology in the measurement of the surface flow velocity in the river channel. By setting up a camera above the river channel to collect the video of the river surface, the collected video is decomposed into consecutive frame images and subjected to pre-processing such as denoising and projection transformation, and the time-space image velocity measurement method is used to analyze the flow velocity. Through the experiment, the error of the identified result of the time-space image velocity measurement method is within less than 5%, and the calculation frequency is 5 min/time. Compared with traditional hydrological measurement equipment, the image method for measuring flow velocity technology can provide accurate and real-time flow velocity data under specific conditions, which provides strong support for flood monitoring and early warning.