The shielding effect of polyethylene (PE) coating disbondment on cathodic protection (CP) permeability was investigated. Results demonstrated that CP became shielded by disbonded PE coating. The geometrical factors of coating holiday (i.e., holiday size) and coating disbondment (i.e., disbonding width and depth) played an essential role in CP permeability. When sulfate-reducing bacteria (SRB) were contained in the thin layer of solution trapped under the disbonded PE coating, the corrosion of the steel depended heavily on the thickness of the solution layer. There existed a critical solution layer thickness, i.e., 150 μm, where a maximum corrosion rate was reached. Pipeline corrosion in SRB-containing thin layers of solution trapped under disbonded coating was resulted from the synergism of microbiologically influenced corrosion (MIC) and the formation of corrosion product film on the steel surface.