To explore the effectiveness and technical features of inspecting the wharf joint underwater by multi-beam bathymetric system,we carry out an underwater field experiment on caisson quay wall of the wharf being built.According to the characteristic of the object,we improve the configuration and installation mode of the multi-beam bathymetric system,and analyze the point cloud extracted from the data collected,and give the verification for the width of the joint by underwater photography.The results show that the resolution of inspecting joint width is related to ship speed,launch frequency of beam,detection distance and other factors,and steady navigation attitude and uniform speed should be ensured to guarantee the quality of inspection.This system can obtain three-dimensional point cloud data underwater,which can directly reflect the overall shape of the joint.It can provide a new method for inspecting wharf joint underwater.