The invention discloses a dynamic display system for a streak line in water flow and a display method for the streak line. The system comprises a river engineering model, streak line trace particles, a camera and a digital image processing device. The method includes: spreading the streak line trace particles in the water flow; shooting the motion video of the trace particles by the aid of the camera; extracting single-frame particle images by means of processing digital images; extracting particle images by means of image segmentation; marking the positions of the trace particles in each frame of images in a current background image with a certain conspicuous color to form streak lines; and finally, converting each frame of processed digital images into a continuously played video to dynamically display the motion process of the streak line. The system is simple in structure, easy in setup and beneficial to popularization and use, and the method not only can be used for clearly displaying the streak line, but also can be used for dynamically displaying the change process of the streak line, and provides technical support for flood prevention, drought resistance, environmental protection and the like.