In order to fundamentally understand the effect and mechanistic aspects of non-stable cathodic polarization (NSCP) on stress corrosion cracking (SCC) of pipeline steels, square wave polarization (SWP) was employed to investigate the interference of NSCP on SCC behavior of an X80 pipeline steel in a near-neutral pH environment. It was found that alterative stages of SWP, i.e., pulse current and stable current period, play different roles in SCC. The former mainly contributes to the local anodic dissolution (AD) process, while the latter strengthens the process of cathodic hydrogen evolution, thus resulting in increased susceptibility to SCC. Essential electrochemical processes involved above were analyzed in combination of physical and electrochemical models.