Abstract: The rust layer stabilization treatment was carried out on Q500qENH weathering bridge steel samples by combination of coating rust layer stabilizer and water treatment (84 d). The micromorphology, phase composition and formation mechanism of the surface rust layer of the sample were studied. The results show that the surface corrosion of the weathering bridge steel sample with rust layer stabilizer was uniform in the early stage of water treatment, and the rust layer formed in the later stage had good compactness and no obvious cracks inside. After water treatment for 84 d, the rust layers on the surface of the samples with and without rust layer stabilizer were both mainly composed of α-FeOOH, γ-FeOOH and a small amount of Fe₃O₄, and the mass ratios of α-FeOOH to γ-FeOOH were 2.77 and 1.85, respectively; the surface of the sample with rust layer stabilizer preferentially formed a stable rust layer. The rust layer stabilizer could make the weathering bridge steel quickly form a uniform rust layer, accelerate the formation and transformation of the initial phase γ-FeOOH in the rust layer, increase the content of the stable phase α-FeOOH in the rust layer, promote the formation of the stable rust layer, and shorten the stabilization period of the rust layer.