Abstract: The rust layer is a critical factor in determining the corrosion resistance performance of weathering bridge steel. Understanding the evolution mechanism of this rust layer is fundamental for the design and optimization of such steel. This study investigates the evolution of the rust layer on high-Cr-content weathering bridge steel, using an atmospheric corrosion monitoring (ACM) sensor and big data mining techniques in a simulated tropical marine atmosphere. Results reveal that the protective properties of the rust layer follow a periodic pattern of “ascending–constant” rather than a continuous ascending. Correlation analysis indicates that this phenomenon is attributed to the introduction of Cr, which promotes the formation of FeCr₂O₄ in the rust layer. FeCr₂O₄ helps prevent chloride ions from penetrating the rust layer, exerting a protective effect. These findings provide a strong scientific foundation for the design and improvement of new high-Cr-content weathering bridge steels.