In order to achieve intelligent real-time monitoring of weak currents, the magneto-optical crystal is used as the sensing element of fiber optic current sensor based on the Faraday effect with the advantages of the large Verde constant, high sensitivity, small size, easy integration and so on. The demodulation mechanism of a polarization fiber optic current sensor using magneto optical crystal is derived and analyzed with the Jones matrix. A polarization fiber optic current sensing system for measuring weak current using magneto-optical crystal is built. And a dual optical self balancing measurement method using the basis of polarization analysis technology is proposed to eliminate the unequal loss coefficients of the two optical paths, and the demodulation expression for polarization fiber optic current sensor in practical applications is obtained with a maximum sensitivity of 7.15×10^-4rad/mA and a micro current measurement resolution better than 0.5mArms.