A new kind of power frequency current mutual inductance device based on the optical fiber measurement technology was proposed in this paper. The new device was made up of excitation current source, sensing system and magnetic circuit. The bus-bar was selected as excitation current source. The giant magnetostrictive material and fiber Bragg Grating (GMM-FBG) were combined as the sensing system. The magnetic circuit was consisting of the magnetic materials and permanent magnet materials. The numerical analysis of the magnetic field which generated in the sensors was achieved through the finite element analysis. In the simulation, the quiescent operating point of GMM-FBG sensor was confirmed; the size of the material in the magnetic circuit system was optimized to ensure the good output performance of sensing system. Finally, the numerical relationship between excitation current and the internal magnetic field of the Terfenol-D rods was established. Meanwhile, the calibration of sensor was achieved through the experiment system of the GMM-FBG current transformer sensor.