As one of the common measurement equipment of primary side current, the current transformer plays an important role in AC current measurement, relay protection, power equipment maintenance control and other related fields of the power system. To address the problems of low linearity, poor anti-interference ability and low precision existing in traditional current transformers, it is especially difficult to accurately measure the low power frequency current. In view of this, a fiber optic current transformer based on the MachZehnder interferometer is proposed in this article. The transformer uses the Joule heat generated by the current passing through the resistance to change the optical signal characteristics in the fiber to cause the change of the output signal of the Mach-Zehnder interferometer. Then, the optical signal of the interferometer is captured by the photodetector and converted into electrical signal output to complete the power frequency small current detection. In the structure, the PCB-type Rogowski coil is adopted as the induction device, and its dynamic response analysis is completed by MATLAB. In addition, a signal processing system based on FPGA technology is designed to improve the signal to noise ratio. Finally, the small power frequency current is measured through the experiment. The linear correlation coefficient is 0. 996 1, and the detection accuracy can reach 0. 14% . The experimental results show that the proposed fiber optic current transformer has higher linearity and measurement accuracy than the traditional optical current transformer, which provides a new idea for the development of current transformers based on thermal effect. It also provides a new method for the measurement of power frequency small current.