Abstract: It's one of the most extensive methods to inspect workpiece with X-ray and automatic inspection based on image processing is an important field. The imaging model and calibration establish a geometrical relationship between workpiece and its image, which is the precondition for automatic orientation and measurement with images. Ignored some factors such as the shape of the image intensifier's input screen, the paper develops a model and calibration method for the imaging system used in automatic nondestructive testing system, which is used in weld defect inspection. The results show that the method is effective.