An optical fiber magnetic field sensor was proposed and demonstrated. The sensor head is composed of a Mach-Zehnder Interferometer (MZI) cascading with a Fiber Bragg grating (FBG). The MZI consists of a taper structure and a peanut-shape structure, which are encapsulated in capillary with Magnetic Fluid (MF) . The taper structure and peanut-shape structure is equivalent to an optical splitter and a coupler respectively. The external magnetic field intensity can be measured by the variation of characteristic wavelength as the effective refractive index of MF changes with the variation of magnetic field intensity. The transmission peak of FBG is insensitive to magnetic field intensity, the experimental results show that, when the magnetic field intensity varies from 0mT to 20 mT, the magnetic field sensitivity is 0.11 nm/mT. Experimental results show that, the temperature sensitivities of the interference peak of MZI and the transmission peak of FBG are 0.401 5 nm/℃ and 0.011 4 nm/℃, respectively. Therefore, the simultaneous measurement of the magnetic field intensity and temperature is demonstrated based on the sensitive matrix.