Elliptical degeneration of output light of optical current transformer is caused by linear birefringence and refection-induced retardance, which results in reduction of sensitivity. Faraday mirror-typed optical current transformer (FMOCT) can effectively solve this problem. Non-polarizing beam splitter (NPBS) is the core apparatus in this project. In order to exactly master the capability and function of NPBS, matrix model of NPBS and Faraday mirror-typed optical current transformer is built by Jones matrix. The influence of reflection-induced-retardance of NPBS upon the polarization state of the output light of FMOCT is analyzed. Then the project of measuring the value of reflection-induced-retardance of NPBS whose actual value is 5.02°under normal temperature is proposed and experimental temperature curve is drawn. The curve of polarization state of output light of system is simulated by the software MathCAD and the relation between elliptical ratio and reflection-induced-retardance is analyzed. Drift range of reflection-induced-retardance which is 0.2 grade allowed in metering level of optical current transformer is less than 0.1%. The result might have reference on the practical application investigation of FMOCT.