Abstract: The cables in anchorage zone of cable-stayed bridge are hidden within the embedded pipe, which leads to the difficulty for detecting the damage of the cables with visual inspection. A detection device based on high-resolution video capture was built, the distance observing of invisible segment of stay cable and damage detection of outer surface of cable in the small volume were realized. The system mainly consisted of optical stents and precision mechanical support device, optical imaging system, lighting source, driving motor control and IP camera video capture system. A set of telescope objectives with three different focal lengths were designed and used in different distances of the monitors by means of converter. Lens system was far separated from lighting system, so that the imaging optical path could effectively avoid the harsh environment which was in the invisible part of cables. The practice shows that the device not only can collect the clear surveillance video images of outer surface of cable effectively, but also has a broad application prospect in security warning of the prestressed structures.