Abstract: This paper discusses a type of ultrasonic testing technology, Time-of-Flight-Diffraction(TOFD for short), to detect the special welding defects vertical to the test surface. This technology focalizes on the recognition and visual detection of welding defects in Friction Stir Welding(FSW for short), and carry out the assessment methods of TOFD on FSW. Through weld NDT research, we establish FSW defect discrimination method to optimize the welding parameters and improve weld quality. This is good for the promotion and application of FSW.