Based on the principle of FBG sensing mechanism, an alternating current sensor system, which uses a FBG attached on a magnetostrictive rod as the probe, was developed. A dynamic interrogation method was proposed based on wide band light source, by making use of the linear relationship between its power intensity and wavelength as spectrum filter. A 50 Hz AC signal is interrogated successfully through the developed system and the system transfer function was established. The amplitude and frequency of the AC signal to be measured can be well deduced through the transfer function of the system, so the alternating current measurement is accomplished.