A multi-beam bathymetry system was introduced from marine surveying and mapping to the hydropower station detection industry so that non-destructive testing for the erosion damage effect of the flood energy dissipating system can be carried out. The multi-beam bathymetry system combined with the video filmed by an underwater robot was applied to detect the erosion damage of the cement concrete mulch and side walls of a hydropower station. Through analysis of the testing process and data, the application conditions of no signal shielding was determined. Detailed detection characteristics such as scouring, wearing and caving were summarized and the result shows the multi-beam bathymetry system has good detection effects for erosive wear especially for the faulting of slab ends of which the scope and degree can be clearly estimated. The multi-beam bathymetry system also has good detection effects for erosive caving and the underwater topography around the scouring pit can be accurately plotted with a better estimation for the scouring scope and depth.