Abstract: An ultrasonic nondestructive testing method was presented, which included data acquisition, processing and three-dimensional visualization. Data acquisition was conducted with ultrasonic phased array probe, after the welded joint was scanned and the flaw data were collected. Subsequently, three-dimensional reconstruction was carried out by programming on platform of Matlab. This method was first verified with artificial defects of flat-bottomed holes, then it was applied to friction stir welded (FSW) joint, and the 3D visualization of the joint defects was achieved. The results show that this method increased the objectivity and intuitiveness in NDT, therefore the testers do not have to analyze a lot of two-dimensional images and imagine three-dimensional situation by themselves; the space distribution of the defects can be directly observed from three-dimensional reconstruction results, the morphology and direction of the defects within joints can be easily understood.