In the field of bridge cable inspection using crawling robots, challenges such as difficulty in measuring multiple rope diameters and inaccurate damage assessment persist. These issues stem from the heavy weight of traditional crawling robots, low sensing accuracy, and poor detection versatility. To address these challenges, an open-loop permanent magnet magnetization sensor device based on magnetic flux leakage (MFL) detection was developed. This device achieves high-precision structural damage detection while reducing magnetic attraction through a lightweight design, enabling the robot to flexibly navigate the cable. The sensor’s universal design allows the robot to adapt to varying rope diameters. A kinematic structure for the flexible-drive robot was established, incorporating a flexible mechanical design with a high thrust-to-weight ratio, allowing the robot to efficiently crawl on cables of different diameters. This design enhances detection efficiency and improves the robot's adaptability in complex environments. An open-loop permanent magnetization-based cable robot detection system was established. By synergizing magnetic information capture, hardware systems, and flexible drives, the robot can autonomously detect damaged structures. Through the synergy of magnetic information capture, hardware systems, and flexible drives, the autonomous in-line-inspection (ILI) robot conducts non-destructive testing (NDT) of cables. Finally, a test was conducted on the internal and external damage of a cable with a diameter of 28 mm, and the results show that the crawling robot can meet practical inspection requirements.