Abstract: In order to study the application of ultrasonic nondestructive (NDT) detection in weld defects identification of metal structures in hydraulic engineering, three methods:conventional ultrasonic inspection method, ultrasonic phased array technology and time of flight diffraction (TOFD) technique, are used to identify the defects in welded samples of metal structures. The characteristics of various defects detected by the three methods are analyzed. The results indicate that the conventional ultrasonic inspection method and TOFD technique can display a variety of defect signals; the ultrasonic phased array technology can effectively detect these defects except porosity and transverse cracks; the conventional ultrasonic inspection method is difficult to determine the shape of defect; in TOFD inspection, the porosity and transverse cracks show special arcs, the signals of the internal crack with a certain height and the incomplete fusion are composed of diffraction waves at the up and down ends, and the diffraction waves at the up and down ends of incomplete root penetration are not obvious. The detection results of ultrasonic phased array technology are intuitive and can be used to measure burial depth, self height and the length of defects, however, the detection rate is very low when scanning spot defects or crack defects parallel to ultrasonic beam.