The effect of pH value on the stress corrosion cracking (SCC) of API X70 pipeline steel in simulated acidic soil solutions was investigated by using slow strain rate test, electrochemical polarization curves, electrochemical impedance spectroscopy, and scanning electron microscopy. pH plays an important role in the susceptibility and electrochemical mechanism of SCC. The pH higher than 5 has no significant effect on electrochemical processes. By contrast, the pH lower than 5 intensifies cathodic hydrogen evolution reactions, thus increasing the cathodic current and corrosion potential. Under different pH values, the SCC mechanism of X70 pipeline steel varies among anodic dissolution (AD), hydrogen embrittlement (HE), and the combination of AD and HE (AD + HE) with variations of applied potential. At −850 mVSCE, the SCC mechanism is HE if pH is less than 4 or AD + HE if pH value is more positive.