Abstract: Rubber-lined pipelines are widely used in the three-loop system of nuclear power plants due to their good corrosion resistance and excellent economy. However, the rubber lining structure may suffer from damage, peeling, and bubbling after a period of service, leading to the corrosion perforation of metal pipes. However, there is currently a lack of a suitable nondestructive testing method. Based on the three-axis fluxgate sensor, the surface magnetic induction intensity distribution of the rubber-lined steel plate with defects was studied, and the magnetic flux density distribution at rubber-lined defects based on COMSOL simulation was studied. The research results showed that the three-axis fluxgate sensor had a certain response to the damage of the rubber lining, and the magnetic flux density distribution at the rubber-lined defect was distinctly different from that of the normal area. The data measured by the fluxgate sensor was processed, and the results showed that effective data processing could increase the recognition of the defect signal.