By the way of computer simulation,the influence of bias angle of Faraday mirror upon polarization state of output light of Faraday Mirror-typed Optical Current Transformer is analyzed using the theory of Jones Matrix. The value of actual bias angle of Faraday mirror and the effect of out magnetic field upon bias angle of Faraday mirror are measured by experimentally. Existence and changes of out magnetic field cause bias angle of Faraday mirror deviating initial value and output light of system degenerating into elliptical polarization light from ideal linear polarization light. It brings the decline of sensitivity and stability of FMOCT. At the same time solving the influence of out magnetic field by electromagnetic shield is proposed.