Abstract: An optical fiber magnetic field sensor is proposed and experimentally demonstrated by using a U-shape cavity based on in-fiber Mach–Zehnder interferometer (MZI) coated by magnetic fluid (MF). The magnetic field sensor is fabricated by splicing a section of single-mode fiber (SMF) between two sections of SMF with designed fiber geometry relationship. As the geometric symmetry MZI is strongly sensitive to the surrounding refractive index (RI) with a high sensitivity up to -13588nm/RIU and MF’s RI is sensitive to magnetic field, the magnetic field sensing function of the proposed structure is realized. The results show that the magnetic field sensitivity reaches up to 137pm/Oe with magnetic field range from 0 to 250 Oe almost linearly. Our magnetic field measurement structure has a relatively advantages of small size, low cost, easy fabrication, robustness, high sensitivity, good repeatability and easy integration with fiber systems.