In this work, effect of alternating current (AC) on corrosion behavior of pipeline steel in the presence of sacrificial anode or grounding were investigated in 4g/L Na2SO4 simulated soil solution by weight loss tests, electrochemical measurements and electrochemical analyses. It is found that the imposed AC shifts both potential of pipeline steel and sacrificial anode (or grounding) away from the value prior to AC. The difference of the offsets results in an enhanced DC current between pipeline and sacrificial anode (or grounding). As a consequence, corrosion of pipeline is accelerated or controlled, which is close related to the electrochemical characteristic of sacrificial anode (or grounding). Based on experiment results, a mixed AC corrosion mechanism considering effect of AC on the interactions between pipeline steel and appurtenances, i.e., sacrificial anode and grounding is proposed.