An optical fiber Sagnac magnetic field sensor based on magnetic fluid is proposed. The magnetic fluid exhibits magneto-optic birefringence under external magnetic field, in which the nanoparticles form chains along the magnetic field and cause the anisotropy. The interference spectrum of the sensor shifts with the external magnetic field due to the magnetic fluid film inserted in the Sagnac loop together with a section of polarization maintaining fiber. The output spectrum shifts with the variation of magnetic field strength when the plane of the magnetic fluid is perpendicular to the direction of the light and parallel to the direction of the external magnetic field. The achieved sensitivity and resolution of the sensor, which are related to the thickness of the magnetic film, are 16.7 pm/Oe (1 Oe≈79.578 A/m) and 0.60 Oe for 60 μm thickness, respectively. When the plane of the magnetic fluid is perpendicular to the direction of the magnetic field, the output spectrum does not shift with the variation of the magnetic field strength.