To address the limitations of manual inspection for suspension bridge main cables (e.g., blind spots, strong subjectivity, and poor safety), as well as the shortcomings of existing robotic systems, including potential damage to protective layers and difficulties in engineering deployment, a robotic system for surface defect inspection based on a bionic inchworm climbing principle was proposed in this paper. First, a bionic inchworm body was designed, which employs two sets of grippers that open and close alternately to grip the handrail ropes as the climbing carrier. A bidirectional synchronous rail-adjustment component was incorporated to enable adaptation to cable systems with handrail spacings ranging from 700 mm to 1 100 mm. The entire robot was structured with a modular design to facilitate convenient and efficient deployment. A closed-loop cooperative control and image acquisition system, leveraging multi-sensor fusion, was developed to ensure stable climbing on steep inclines. Subsequently, mechanical models for the entire robot and its key components, such as the clamping shoe assembly and the bidirectional synchronous rail-changing component, were established. Targeted mechanical analysis and finite element simulation analysis were conducted. Finally, field tests were carried out on the Cuntan Yangtze River bridge. The experimental results demonstrate that the robot can stably climb from the lowest point of the main cable at the mid-span of the suspension bridge to the bridge tower, and successfully collect panoramic images covering the entire circumference of the main cable. The feasibility of the design scheme and its engineering effectiveness are thus verified. This work provides new equipment for the intelligent operation and maintenance of suspension bridge main cables.