Abstract: Considering the structure of pipes and cost of the inspection, the several times radiation method can not be used in the RT for small diameter welding tubes in utility boiler. According to the related standard, the elliptical RT method was used. Through the theoretical analysis, it was concluded that, the opening gap of welding image was only related to radiation degree, when the diameter, theoretical focus and width of welding were determined. The X-ray testing system was refit with a self-designed goniometer, and was used in the real testing. The result showed that, the opening gap of welding image was guaranteed and the detection rate of defects in root were increased. The goniometer was flexible and convenient to use, decreasing the measuring time and increasing the working efficiency, and was worth while popularizing.