Abstract: In this paper, the fretting wear mechanism of the thimble tube of neutron flux measurement in the core of the CPR1000 nuclear power plant is briefly described. For the common fretting wear at the support, an accurate measurement of the wear length using the eddy current method is investigated. The coil response range that mainly affects the measurement results is analyzed and calculated. Accurate outer wall wear length measurement results provide an important reference for the design of the casing and the follow-up maintenance.