The MOCT(magneto-optical current transformer) is analyzed ,which is used in smart substation for power quality current data measurements. It is found that the output signals of the MOCTs are influenced by both the fluctuation of operating wavelength and the intersecting angle error of optical axes of polarizer and analyzer. The process of a mathematical model of magneto-optical current transformer is discussed in detail, first magneto-optical current transformer sensor head Jones matrix is established, An overall model is established under ideal conditions and the actual conditions based on an improved dual optical paths detection method. The measurement performance of harmonic current transformer signal disturbance is discussed based on ideal conditions and the actual situation.