In response to the demand of rapid measurement of water surface boundary in river model test, an instrument based on the laser scanning principle is developed. The instrument adopts high-resolution line matrix CCD and laser triangulation principle to detect and identify the water surface rapidly. It has been successfully applied to the synchronization acquisition of a wide range of flow velocity in physical river model. The instrument has advantages of high speed, high precision, and non-contact measuring, hence could be a feasible alternative for rapid measurement of water surface boundary in physical river model.