The invention discloses a method for implementing online tests of stream flow based on video images, and belongs to the technical field of non-contact open channel flow measurement. The method relates to a water flow tracing system, a water surface illuminating system, a video flow acquisition system, a camera calibrating system, an image processing system and a flow estimating system, trace particles are thrown in a flow measuring stream segment by the water flow tracing system, a camera shoots a water surface video stream, frames are extracted at fixed intervals and are processed digitally, the image processing system processes digital images according to ground control point coordinate information provided by the camera calibrating system, and finally, the stream flow is computed by the aid of the flow estimating system according to a flow rate and area method. By the aid of the method, complicity of stream flow measurement is reduced, simultaneously, high measurement precision can be guaranteed, and the stream flow can be dynamically monitored in extreme environments.