A tapered optical fiber magnetic field sensor for measuring the power frequency magnetic field was proposed in this article, which could be placed inside the transformer tank and realize the measurement of the power frequency magnetic field. The proposed sensor used magnetic fluid (MF) as the magnetic sensitive medium. The change of the MF’s refractive index changed with magnetic flux density, causing the change of the transmitted light intensity in the transmission process. Firstly, the dynamic response characteristics of magnetic field sensor in 50Hz power frequency magnetic field were investigated. The results showed that the output waveform was a double frequency of the magnetic field. The dynamic response amplitude increased linearly with the magnetic flux density in the range of 0–20 mT. When the external magnetic field intensity was greater than 20 mT, the sensor reached saturation. Then, the cross-sensitivity of temperature and magnetic field was investigated. By adding reference arm and solving cross-sensitivity matrix, the cross sensitivity between temperature and magnetic field was solved.