An optical current transformer is constructed by exposing gain mangetostrictive material (GMM)-FBG system to current-induced magnetic field. Ferro-magnetic materials are used to design a loop to constraint and conduct the flux into GMM. The permanent magnetic material is used in loop to establish a bias flux to form a working bias for the system. Finite element method is used to analyze and design the loop structure and the size. Coarse wavelength division multiplexing (CWDM) device is used to demodulate the alternating strain on FBG caused wavelength drift to realize sensing of the current. In the experiment,the maximum current is 186 A and the full scale accuracy is 4.3% when the system workes in linear region by setting the bias in 30 kA/m. Fast Fourier transform algorithm (FFT) is used to analyze the harmonic component of the output signal to evaluate the quality. It is demonstrated that the output signal is not influenced by hysteresis and nonlinear effect of GMM in linear region.