In view of the characteristics of the small amplitude of a current signal, an all-optical fi bre small-current sensor for partial discharge is devised. A novel type of sensor head with electric winding light is proposed. A complete optical model of the optical fi bre current sensor is developed under the eff ect of polarization error factors. In addition, the eff ect of polarization error factors on the optical fi bre sensing system is simulated. The simulation results show that as the extinction ratio increases, the output relative error of the optical fi bre small-current sensor decreases. The eff ect of the fast and slow axis misalignment errors of a polarizing beam splitter is similar to the eff ect of the alignment angle error of an optical fi bre polarizer. It is observed that the relative error curve follows a sine function. Meanwhile, the output relative error increases fi rst and then oscillates with an increase in the linear birefringence. We conclude that the linear birefringence of the sensing optical fi bre is the main error source for the measurement accuracy of the sensing system. Finally, an experimental system for the optical fi bre small-current sensor is implemented. The experimental results show that the eff ect of linear birefringence on the fi bre current sensing system can be suppressed by introducing a large amount of circular birefringence into the sensing optical fi bre. The theoretical mechanism of these errors is analysed, which can be applied to implement corresponding measures to reduce the eff ect of error factors on the sensing system and further improve the measurement accuracy of the optical fi bre small-current sensor.