A method to determine the phase difference in a 3×3 fiber coupler by the all fiber optic current transducer was presented. The theoretical model of the response of an all fiber optic current transducer, according to the principle of the Sagnac interferometer, to the parameters of phase difference of a 3×3 fiber coupler and the interference fringe was established. An objective function was constructed by calculating the sum of square error of the measured response and the predicted response with different couples of unknown parameters. An optimization algorithm was used to obtain the phase difference and the interference fringe. Experimental research was carried out on a commercial 3×3 coupler, and the measurement result agrees well with the theoretical estimation.