It is known that calcareous deposit formed on the surface of metallic material may have effect on the adhesion of marine micro-organism to and the corrosion behavior of the substrate material. Meanwhile, micro-organisms may in turn affect the formation process of calcareous layer. To clearly understand the above mentioned phenomena, the electrochemical behavior of the Q235 carbon steel without and with a pre-deposited calcareous layer in the f/2 culture media with the absence and presence of chlorella vulgaris respectively, as well as, the adhesion behavior of chlorella vulgaris on the substrate were investigated by using fluorescence microscopy, surface analysis techniques and electrochemical measure method. Results show that the adhesion process of chlorella vulgaris reached the equilibrium phase of adsorption-desorption after immersion of the steel in the medium for 48 h. Absorption is the decisive step in the initial attachment stage of chlorella vulgaris on the steel with calcareous deposit, and then the adherent microalgae began to proliferate. The calcareous deposit could promote the adhesion of microalgae, however, the calcareous deposit was apt to spall off in the f/2 culture medium without chlorella vulgaris. During the immersion process, composite films of calcareous deposit and microalgae may formed on the steel surface, which was compact with good adhesion to the substrate and presented blockage effect to the charge transfer and inward diffusion of oxygen, thereby can effectively inhibit the corrosion of the steel.