According to the high-voltage and small-current characteristics for unbalanced current of high-voltage capacitors, a novel fiber optical filter unbalanced current transformer (FU-FOCT) is proposed, where an all-fiber temperature sensor based on the temperature birefringence effect of polarization-maintaining fiber is designed and integrated into an FU-FOCT for real-time temperature correction. The FU-FOCT prototype with a rated current of 1 A and a rated voltage of 258 kV is developed, and the accuracy, temperature, and frequency-response characteristics are tested. The results show that within the temperature range of ${-}{40}^\circ {\rm C} - 70^\circ {\rm C}$, the fluctuation range of the ratio error and phase error of the FU-FOCT at 1 A are ${-}{0.042}\% - 0.08\%$ and ${-}{0.16^\prime - 0.18^\prime}$, respectively, which meet the error limit requirements of class 0.2% specified in GB/T 20840.8-2007.