Abstract: The topology and operational principle of fiber optic current transducer(FOCT) were introduced, and the /4 wave plate affecting the performance of FOCT were analyzed. The fabrication of /4 wave plate was studied and several factors that affect the fabrication were introduced, according to the analysis, the improved /4 wave plate that meet the claims is manufactured by using suitable material and method. The experimental results show that the FOCT's measurement precision is improved by using the improved /4 wave plate, and the FOCT can satisfy the accuracy requirements of the 0.2S class can be satisfied by the FOCT, meeting the requirements of digital substation.