During the nondestructive testing and periodic inspection of special equipment like pressure vessels and pressure pipes, the plant must be first shut down to do process treatment for those special equipment and then nondestructive testing can be done. In order to increase detection efficiency and defect detection rate, and reduce inspection cost, time of flight diffraction (TOFD) ultrasonic testing was used to test overall for the special equipment. The method not only is more reliable than other methods for detecting buried defects, but also greatly reduces the cost of inspection because it can be carried out while equipment is running. Due to structure problem of scanning rack, TOFD is mainly applied to inspection of flat welding seams and circumferential weld seams of pressure equipment with large diameters , but it cannot be applied to inspection of plane welded seams and thick-wall pipes with small diameters.Through structure modification of the scanning rack, TOFD can be well applied to inspection of non -planar welded seams and thick-wall pipes with small diameters.