We computed and discussed the impact on the all-fiber optic current transducer’s (AFOCT) output characteristic of the sensing components. The null-drift and measurement deviation of two AFOCT systems with different reflecting mirror are studied. At last, we verifies the rationality of a method to eliminate the interference of linear birefringence by applying circular birefringence on the sensing fiber. The retardation of the circular birefringence applied on the sensing fiber to satisfy the national standard is given.