Abstract: Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and electrochemical methods were used to study the effect of navicula on the corrosion behavior of Q235 carbon steel. The results showed that the surface of Q235 carbon steel was uniformly corroded after immersion in the culture solution without navicula, and severe localized corrosion occurred on the surface of Q235 carbon steel after immersion in the culture solution containing navicula. The navicula attaching to the surface of carbon steel could secrete extracellular polymer (EPS). At the beginning of the experiment, the biofilm had a certain protective effect on carbon steel. Photosynthesis was carried out on the surface of carbon steel. The local biofilm detachment caused the localized dissolved oxygen content be too high, the cathodic oxygen reduction current became larger, and thus accelerated corrosion.