A self-designed experiment circuit was adopted to investigate the effect of alternating current (AC) on corrosion rate and cathodic protection (CP) potential of cathodically protected Q235 steel in the simulated soil solution of 4 g/L sodium sulfate. It is found that the corrosion induced by AC current can be controlled by increasing CP current when AC current density is below 300 A/m2. However, in case that CP current is too large the corrosion of Q235 steel will be accelerated. Moreover, CP potential of Q235 steel shifts considerably due to AC current and the shift direction depends on the applied CP current. Based on the experimental results, a combination role of CP current and AC current considering micro-electrons transfer is introduced to explore the mechanism of AC corrosion.