A novel fiber-optic magnetic field sensor with high interrogation speed and resolution by using an etched fiber Bragg grating (FBG) in conjunction with a dual-loop optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. A commercial FBG is firstly dipped into mixed hydrofluoric acid solution to remove the cladding layer and then is embedded with the magnetic fluid (MF) as a sensing element. The central wavelength reflected from the FBG is related to the overall time delay of the dual-loop OEO, which determines the oscillating frequency of the OEO. Therefore, the magnetic field can be estimated by measuring the oscillating frequency shift of OEO. The experimental results show that the oscillating frequency linearly increases with the increment of the magnetic field, achieving the sensitivity of 16.3 Hz/Oe with an R-square of 0.991 in the range of 5 mT-10 mT. In addition, the maximum error is within ±0.05 mT in the range of 7 mT-8 mT, which offers potentials in many fields where the high-precision magnetic field measurement is required.