Verdet constant is one of important parameters which can determine sensitivity of a fiber optical current transducer (FOCT). Calibration signal is calculated based on Sagnac interference principle, and the output signal of FOCT is measured. By comparing above these two sets of signals, an objective function is established, and then the parameters of the function are optimized based on a Nelder-Mead simplex method. Thereby the Verdet constant of silica fiber is measured. Experimental results are in good agreement with the results calculated by classical models. Phase delay is added to the output signal when it goes through the measuring circuits. Influenced by rotator and changing environment, the working point of the interference loop drifts. As a result, the output signal becomes asymmetric and creates the direct current quantity. Considering all these situations, the phase delay, working point and direct current quantity which reflects the nonlinear distortion can also be obtained by this method.