A newly developed rust layer on Q370qENH weathering steel (sample C) was studied under dry–wet cycle test of 3.5 wt.% NaCl solution and salt spray test, compared with the bare weathering steel (sample A) and the weathering steel with additive contents (CuSO₄ 0.2, FeSO₄ 0.1, NaHSO₃ 0.02, NaCl 0.02, Na₂HPO₄ 0.01, and water balance, in mass%, termed as sample B). Corrosion mechanism of Q370qENH weathering steel after different surface treatments was investigated by means of surface potential scanning, mass change, polarization curve, and X-ray diffraction. The results of X-ray diffraction indicate that rust of bare weathering steel has rare Fe₃O₄, and a dense oxide layer mainly composed of Fe₃O₄ is formed on rust of both sample B and sample C. The surface potential of sample A gradually increases to - 0.2 V in the dry–wet cycle test, while the surface potential of sample A maintains at about - 0.6 V in the salt spray test. The surface potential of sample B and sample C is higher than that of sample A in the early stage and remains stable during the shortterm accelerated corrosion test. Moreover, the mass change of sample C gradually stabilizes with time in the dry–wet cycle and salt spray test. The corrosion current of the sample A is lower than that of sample C in the initial stage of short-term accelerated corrosion test. However, the corrosion current of sample C is smaller than that of the sample A in the experiment. The main components of the deep rust of sample A are γ-FeOOH and α-FeOOH, while those of the deep rust of sample B and sample C are Fe₃O₄.