Abstract: Aiming at the feature of defects on coiled tubing, this paper presents an in-service detection technology for coiled tubing based on alternating current field measurement(ACFM). The ACFM simulation model of cracks on coiled tubing is set up and the characteristic signal of cracks is extracted. The in-service detection system for defects on coiled tubing is developed and the crack inspection experiments are carried out on coiled tubing specimen. The results show that it can detect and evaluate the defects on coiled tubing quickly and efficiently based on the ACFM system. It provides a new technology for online real-time inspection of coiled tubing in service.