Abstract: In the manufacturing process of pressure vessel equipments, there are many special butt welds structures, some of which are difficult to use the conventional TOFD(Time of flight diffraction) detection process for volume detection. In this paper, the TOFD test for the butt girth weld between cone and cylinder with different thickness was analyzed and studied. The whole process was introduced from the basic theory of TOFD, the design and manufacture of special block, the preparation of TOFD scan plan, the test verification and the analysis of the final test results. The results were satisfactory and can be applied to engineering practice.