The output of electric current sensor employing twisted fibre may suffer from variation of ambient temperature or other perturbations such as pressure and vibration. A new compensation method for such perturbations is proposed and demonstrated in this paper for ac current detection. In the experiment, the shift of the initial bias phase of the current sensor is simulated by changing the azimuthal angle of the linearly polarized light at the fibre input, and the validity of the compensation method is verified experimentally. This method is able to compensate arbitrary shift in sensor bias phase.