Abstract: Because the traditional ultrasonic test tech-nique has some shortcomings such as unreliability and poor re-cording,the ultrasonic TOFD (Time of Flight Diffraction) is em-ployed to detect defect of a thick wall (≥20 mm) red copper weldment.The analysis of the acoustic character of red copper provides a guideline for choosing of ultrasonic transducer.The sensitivity and accuracy of the system was tested and the test process was designed through testing an artificial defects con-tained block.Ultrasonic TOFD was carried on red copper weld-ment,and the non-destructive test results were verified by de-structive testing experiments.The results show that,ultrasonic TOFD is effective in recognition of the dangerous weld cracks with proper test process.