Internal corrosion reduces the pipeline thickness and thus is one of the most predominant causes for pipeline malfunction. In this work, a highly sensitive, magnet-assisted hybrid sensor of fiber Bragg grating (FBG) and extrinsic Fabry-Perot interferometer (EFPI) is proposed, and designed for simultaneous measurement of temperature and pipeline thickness loss. The proposed sensor consists of a cylindrical magnet supported with springs, a reflection mirror fixed on the pipe for EFPI setup and a FBG for temperature measurement. The magnetic force between the magnet and the steel pipe is a function of the pipe wall thickness, which can be transferred to the cavity length (distance between the magnet and the reflection mirror) of the EFPI. The cavity length decreases with steel pipe corrosion due to the increase of the magnetic force. The proposed sensor has the sensitivity of 3.5 µm in thickness measurement and can be installed directly on the existed pipeline without interrupting its operation status.