Abstract: With the development of offshore oil engineering ships, the welding technology of hull structures is diversified. In particular, the quality requirements for butt welds of unequal thickness plates of hull plates are getting higher and higher. Radiographic testing（RT）has a great change in the thickness of the unequal thickness of butt weld. If the selection of the parameters is improper, it is easy to make the radiotherapy density negatives too large, which can affect the sensitivity of RT, and can not guarantee the defects in the workpiece to be detected effectively. Thickness tolerance is an important parameter for the RT of variable cross-section workpiece. The thickness tolerance can be calculated through the function relation of exposure curve and the function relation of film characteristic curve, but it is only theoretical value. This paper mainly discusses how to measure the thickness tolerance of different tube voltages and different films under the certain conditions of exposure and focal length by experimental method and its practical application.