The corrosion performance of the 300M steel was assessed via artificial seawater immersion testing and neutral salt spray testing, aiming to simulate the operation environments, by means of electrochemical methods (i.e. OCP, EIS, PDP) and modern instruments (i.e. SEM+EDS, XRD, CLSM etc.). The results show that 300M ultra high strength steel is sensitive to the change of pH for artificial seawater. With the decrease of pH, the cathodic curve changes from oxygen reduction to hydrogen evolution process control, and the open circuit potential (OCP) moves positively, the charge transfer resistance decreases, and the corrosion current density increases. The fitting results of polarization curve and Nyquist diagram indicate that the decrease of pH leads to the acceleration of corrosion. The corrosion rate calculated by mass loss method shows that the corrosion in salt spray testing is more serious than that in artificial seawater immersion testing. However, in the above two cases the steel all presents characteristics of uniform corrosion. This was further proved by observation results of surface morphology and cross-sectional morphology of the tested steel. The corrosion products thickened with local spallation as the progress of corrosion process. The corrosion products was composed of α-FeOOH, γ-FeOOH, α-Fe₂O₃ and Fe₃O₄, the more the α-FeOOH. The difference in corrosion behavior of the 300M steel by immersion testing and salt spray testing is determined by the corrosion electrochemical reaction process and the deposition process of corrosion products. During the salt spray testing, the steel surface covered by thin electrolyte film with adequate oxygen supply, which promotes the deposition of corrosion products, thus promoting the adsorption of Cl^-, and ultimately accelerating the corrosion.