Abstract: The corrosion behavior of marine Q235 carbon steel in artificial seawater with Phaeodactylum tricornutum (seawater with algae) under cyclic light 12 h/dark 12 h, continuous light, and continuous dark conditions, and the electrochemistry performance after corrosion for 7 days were studied, compared with that in the seawater without algae. The effects of the life activity of Phaeodactylum tricornutum on the corrosion behavior of carbon steel were analyzed. The results show that in the seawater with algae, the depolarization reaction of oxygen was dominant under continuous light condition, resulting in pitting on the carbon steel surface, and the corrosion product was γ-FeOOH. Under the condition of continuous dark, carbon dioxide corrosion caused uniform corrosion of Q235 carbon steel surface, and the corrosion product was FeCO₃. The two types of corrosion methods were combined under the cyclic light 12 h/dark 12 h condition, and the corrosion products consisted of γ-FeOOH, Fe₃O₄ and FeCO₃. In the seawater with algae, the charge transfer resistance of carbon steel decreased and the self-corrosion current density increased after immersion under different light conditions for 7 days, compared with that in the seawater without algae.