Abstract: The coating of the bridge cable is a bit thick. It is difficult to overcome the shortcomings of low efficiency and lift-off effect while applying the methods of ultrasound and magnetic flux leakage to detect the corrosion and fracture on line. Aimed to overcome the limitation of the present technology, a detection method based on X-ray digital imaging technology was put forward. It made use of X-ray digital detector to acquire images along the cable. By programming with MATLAB software, the algorithms of X-ray image pre-processing were studied. It calculated the size and location of the defect by the methods of identification, extraction and area size of corrosion defect.