Abstract: The error of commercial fiber optic current transducer (FOCT) must be less than 0.2%, the signal processing is a key factor for high-accuracy current sensing. The topology, operational principle, technical advantage, signal characteristics of fiber optic current transducer was introduced. A new scheme based on digital open-loop detection for the measurement system of the FOCT was presented, the working principles and features of this design were analyzed. The experimental results show that the FOCT's measurement precision is improved by using the improved measurement system, and the FOCT can satisfy the accuracy requirements of the 0.2 class measuring electronic transducer, meeting the requirements of digital substation, so it is valuable for solving a great deal of problems, such as the signal processing, measurement accuracy, measurement sensitivity, et al.