Based on Faraday and Sagnac effect, a full optical fiber current sensor (FOCS) is developed in order to monitor the current on transmission line. A Y-shape waveguide is inserted in light path and zigzag modulation signal is used creatively. Two-way reverse transmission light signals in the Sagnac loop appear decoherence in Y-shape waveguide by adjusting the slope of zigzag wave signal, and the current can be calculated accurately. The current in range from 40 A to 140 A is measured. The experimental data is simulated linearly and the correlation coefficient reaches 0.9996. Results show that the experimental data are in good agreement with the actual current.