Abstract: The online magnetic detection device based on earth's magnetic field was developed to deal with the lack of simple and effective detection method for coiled tubing(CT) in service. The detection device adopted a two-sectional articulated structure, and can be directly installed between CT reel and injector head. Moreover, it connected to the stripper that can realize quick installation and dismantling. During the experiment, the minimum 0.1mm crack can be identified and the relative error of ovality is less than 3% by monitoring the deformation of the coiled tubing. The CT online magnetic detection device was used in an accumulated detection of over 60000 m CT on the spot and the detection accuracy was over 95%. The results indicated that the device has good reliability and can meet the needs of the online field detection of CT defects.