The corrosion behavior of low alloy steel Q345D and carbon steel Q235B in marine tidal zone is investigated by outdoor corrosion test for 2 years in Qingdao sea area located on the east coast of China. The depth losses of steels exposed for 1 year and 2 year show that the average corrosion rates of Q345D are lower than that of the carbon steel Q235B in marine tidal zone, and the structure observation and composition of rust layers for two kinds of steels are close. Compared with Q235B, the corrosion products formed on Q345D is more compact and uniform. Moreover, the early corrosion behavior of Q345D and Q235B is studied by laboratory cyclic-immersion corrosion test for a period of 30 days in artificial seawater, and the regression analysis is conducted to study the process of corrosion. The results show that the corrosion tendency of Q345D is similar to that of Q235B in the initial stage, because the electrolyte can contact the substrate directly, the corrosion rates of two kinds of steels are higher. As the time is going on, the Q345D steel exerts a better resistance to corrosion than the carbon steel Q235B, and the average corrosion rate is reduced obviously. In addition, the results of electrochemical impedance spectroscopic (EIS) and local electrochemical impedance spectroscopic (LEIS) indicate that the resistance of rust layer on Q345D is higher than that on Q235B, furthermore, a stable rust layer could form easily for the Q345D in artificial seawater.