The corrosion behavior of a new low alloy steel Q415NH is investigated by its exposure to alternate conditions of 12min immersion in a mixed NaCl and NaHSO₃ solution and 48min drying at 45℃ to provide a basis for developing new weathering steels and studying the effects of alloy elements on the corrosion resistance under high Cl^- atmospheric conditions. The results suggest that the corrosion resistance of Q415NH is much better than Q420, and the anticorrosion mechanism can be summarized into the following three aspects: (1) Due to the addition of 3%Ni, Q415NH have a microstructure of large number of fine bainites dispersed on ferrite; (2) The addition of Ni can increase the potential of the steel and the resistance of rusts to improve the anodic passivation; (3) The rusts of Q415NH are much more protective by the formation of a compact and fine structure with the anodic ion-selection.