The invention belongs to the technical field of water conservancy measurement, and particularly relates to a real-time dynamic ultrasonic measurement method for a pier scour pit in a model test. In the model test, when an ultrasonic topographic instrument measures the terrain near the pier scour pit, a measuring probe is blocked by a pier and cannot pass, and full-section measurement cannot be achieved. The real-time dynamic ultrasonic measurement method for the pier scour pit in the model test is mainly characterized by comprising the steps that 1, a vertically combined pier is designed to solve the problem that the pier blocks the measuring probe, in the section scanning process, the combined pier can be detached and restored, and the maximum height of the lower pier body is determined based on the operating principle of the ultrasonic topographic instrument in order to reduce the influence of the upper pier body on the water flow structure; 2, in order to solve the problem of local terrain measurement distortion caused by the fact that the top face of the lower pier body reflects sound waves, a vertical pier inclined connecting method is provided, and the inclination angle is designed according to the beam angle of the ultrasonic topographic instrument. According to the method, real-time dynamic measurement on the pier scour pit is achieved.