A method to measure the magnetic field inside the Giant Magnetostrictive Actuator (GMA) is proposed. Improved Preisach Model is applied to eliminate the unavoidable nonlinearity error in the measurement. The effect of temperature on measurement can be eliminated by double Fiber Bragg Grating (FBG) structure. The double-sagnac loop combined with Polarization Maintaining Fiber (PMF) is used to demodulate the center wavelength. Sensing model of the FBG is established to realize temperature compensation for the measurement system. Experiment results show that resolution of 2.1×10-4 T can be achieved over a range of 127 mT, and the repeatability of overall measurement is 0.227%. Therefore, the proposed method can be used to measure the internal magnetic field of GMA effectively and reliably.