The failure mechanism and the transportation behavior of water of a polypropylene coating in 3.5wt.%NaCl solution was investigated by electrochemical impedance spectroscopy (EIS). The EIS data were fitted by equivalent circuits, the corrosion protection performance in different immersing time was obtained, and the diffusion rate of water in the coating was calculated. The results indicated that the corrosion protection performance of the polypropylene coating was rapidly weakened after immersed in the 3.5wt.% NaCl solution. The resistance of the coating dramatically dropped from 5.33×109 Ω.cm2 to 1.08×109 Ω.cm2 after immersed for 6 days, and then became stable. Based on the relation between the capacitance of coating and immersing time under 10 kHz, the water had uniform penetration in the coating at the early stage, and its transportation behavior followed the second Fick’s law, which was a complex unsteady process. The diffusion coefficient was 3.12×10–11 cm2/s. The time of homogeneous diffusion of water in the coating is about 144 h, consistent with the time for the resistance of coating dropped down to a stable value. At this time, the water absorptivity of the coating was 8.25wt.%. When the polypropylene resin coating was immersed in NaCl solution, the infiltration of water and its transportation to the metal substrate was the main reason to lead the failure of the polypropylene coating. When the water transportation did not follow the the second Fick’s law, the water in the coating reached saturation. At that time, the protection performance of the coating was hugely declined, and the corrosion reaction has begun to occur at the interface. It could be known from this study that the EIS could be well used in detecting the diffision of water through coating quantitatively. This could be used in evalating the protection performance of coating, preticting the service life and analysing failure mechanism.