The electrochemical and stress corrosion cracking (SCC) behavior of API X70 pipeline steel under a disbonded coating in artificial seawater was studied using in situ electrochemical measurements and ex situ slow strain rate tensile (SSRT) tests. Results indicated that the electrochemical behavior differs significantly with distance from defect opening. SCC mechanism of X70 steel under the disbonded coating was typically controlled by the synergistic action of anodic dissolution (AD) and the effect of hydrogen, and the susceptibility to SCC at the bottom of the crevice was much higher than that close to the defect opening. Furthermore, the effect of hydrogen on the SCC process is discussed in particular using a quantitative relationship between the cathodic current density and SCC susceptibility.