To deal with the problem of complex large current measurement site in electrolytic aluminum industry, a portable fiberoptic current transformer was proposed on the basis of the traditional fiber opticcurrent transformerin. The Faraday phase shift error which caused by the flexible sensing coil non-closed optical path and current conductor eccentric position in the process of design and installation was analyzed and calculated by finite element method. It has been obtained that the Faraday phase shift relative error increases linearly with the non-closed angle of sensing coil and decreases with the increasing distance between conductor and the non-closed point, and the Faraday phase shift relative error can be decreased by increasing number of turns. Based the accuracy of portable fiberoptic current transformer increases with the distance between conductor and the non-closed point, a sensor head structure with the extending non-closed point and easy accessibility was designed, it has been obtained that the full temperature accuracy is 0.86% of portable fiberoptic current transformer by experimental test, which meets the required accuracy 1% in smelter.