Q420NH steel surface was stabilized by using stabilizing treatment agent with main components of NaHSO3, FeSO4, CuSO4, NiSO4 and Na2HPO4, and the microstructure and formation mechanism of the surface rust layer on the stabilized and un-stabilized surfaces of Q420NH steel were comparatively investigated by the alternating wet dry cyclic corrosion test (CCT), and by the means of analytical tests such as SEM, EDS and XRD. The microstructure and formation mechanism of the rust layer on the surface of Q420NH steel after stabilization treatment were compared with those without stabilization treatment by means of SEM and XRD. The results show that in the 64-cycle corrosion experiment, the instantaneous corrosion rate of the stabilized specimen was higher than that of the unstabilized specimen. The main composition of the rust layer of the stabilized specimen is mainly α-FeOOH, γ-FeOOH, β-FeOOH, and Fe3O4/γ-Fe2O3, and in the 64-cycle corrosion experiment, the content of α-FeOOH in the rust layer of the stabilized specimen was 28%, which increased by 50% compared with that of the unstabilized specimen, and the content of α-FeOOH in the rust layer of the stabilized specimen was 28%, which increased by 50% compared with the unstabilized specimen. At 64 cycles, the content of α-FeOOH in the rust layer of the stabilized specimen was 28%, which increased by 50% compared with that of the untreated specimen. In the rust layer of the stabilized weathering steel specimen, the elements of Cr and Ni were obviously enriched in the combination of the rust layer with the substrate and inside the rust layer, and the self-corrosion potential of the rust layer was increased from -0.619 V in the untreated specimen to -0.530 V in the stabilized specimen, and the resistance of the rust layer was increased from 172 Ω·cm2 to 365 Ω·cm2. Therefore, stabilization is has a good effect on promoting the formation of protective rust layer on the surface of weathering steel.