The corrosion behaviors and corrosion resistance of Cu-P-RE weathering steels and plain carbon steel Q235 were studied by dry-wet cycle immersion test. A weight-loss method was used to measure the corrosion rates of all steel samples. The results show the corrosion rates of Cu-P-RE weathering steels are much lower than that of Q235 steel. The corrosion resistance of the weathering steels with different rare earth contents is distinguishing. The corrosion resistance of rust layer on the weathering steels and their anticorrosive mechanism were investigated by the electrochemical means of polarization test and electrochemical impedance spectra technique (EIS). The results indicate that the electrochemical means can be used to analyze characteristics of the rust layer on the weathering steels effectively. The 10PCuRE steels have excellent corrosion-resistant performance because of their positive corrosion potentials and large resistance of rust layer. An equivalent circuit was proposed to represent the steel/solution interface reaction process. Some quantitative electrochemical parameters such as rust layer resistance and polarization impedance were deduced from electrochemical impedance spectra of the steels, which can effectively estimate the protective ability of rust layer. There are two different diffusion processes, which are semi-infinite diffusion and finite thickness diffusion, in rust layer on the RE weathering steels. The protective ability of inner rust layer on the weathering steels can be evaluated by the polarization impedance of finite thickness diffusion.