Masses of chlorine salts in the oceans engender a badly electrochemical erosion on the concrete since cross- sea bridge and harbor construction has been intensified in recent years, which has aroused a wide concern. This paper aims to study the corrosion mechanism of seawater chloride ion on the concrete with upper and lower two layers of intertidal zones set up according to different positions. A test is designed to make a comparative analysis on the resistance of marine biofilm to chloride permeability in two cases that the biofilm is active and inactive. The results show that chloride ion takes place electrochemical reaction and plays dual roles of anodic depolarization and conductivity. For its permeability to chloride ions, active biofilm shows stronger, which may be correlated to the halophilic bacteria contained in biofilm and the surface bonding effect of concrete. In relation to the upper intertidal zone, the resistance to chloride penetration in the lower layer of concrete is superior, which reveals that the lower biofilm has a better adhesion effect on the surface and plays a certain role in preventing the erosion of harmful substances.