A new approach is proposed to measure the magnetic field intensity via optical absorption method with optical fiber evanescent wave sensor. A standard single-mode optical fiber used for communication is put into the magnetic field with magnetic fluid and its extramural cladding is local corrupted close to the fiber core. The evanescent wave of the fiber surface is effected by the change of magnetic field, when the absorb peak is used as detection wavelength The change of magnetic field can be obtained by measuring the optical output intensity of the fiber. The experimental results show that there is a linear relationship between the transmitted intensity and magnetic intensity, the magnetic fluid in the concentration range of 40~110 mT, the detection sensitivity respectively are 0.0019 μW/mT and 0.02 μW/mT of 50 μm and 35 μm for the length is 15 cm. The obtained results have instructive function for resistance to interference and easy to implement of optical fiber magnetic field sensor.