Abstract: The corrosion behavior of 2205 duplex stainless steel in the seawater under 170 m of South China Sea was studied by coupon corrosion testing and cyclic polarization testing in combination with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that localized corrosion occurred on the 2205 duplex stainless steel in deep-sea environment. At the initial stage of immersion, a microbial protective film was formed on the surface of the 2205 duplex stainless steel, and the corrosion resistance of the stainless steel improved under the protection of the microbial film. As the immersing time increased, microorganisms died and sloughed off, and then lost the protection for metal substrate, resulting in the decrease of corrosion resistance of the 2205 duplex stainless steel. A gap was formed between the residual microbial film and the stainless steel substrate, and crevice corrosion occurred under the action of dissolved oxygen and Cl^- to form localized corrosion pits.