Abstract: A defect auto-tagging device based on the line scan X-ray testing system was introduced. This device was composed of marking form to write, pneumatic marking pen, linear guides and linear stepper motor. The structure and performance were analyzed from the hardware structure to the motor driving system. It was concluded that high-precision auto-tagging performance, small size, adaptability, etc., of this device, could meet needs for different shape work pieces defect auto-tagging needs.