Abstract: The 660MW ultra supercritical thermal power generating units of thermal power plants demand rather high requirements for the welding level of high temperature reheater, needing a more demanding working environment and stricter standard of testing. This paper shows how to choose the ray source and transillumination position reasonably based on the situation of the site, and to determine the focus and exposure parameters through experiments, and finally it designs and develops the optimum radiographic technology, which is able to meet the requirements of quality and testing efficiency.