Abstract: After steel plants crush steel slag and separate iron from it, they usually conduct simple screening according to particle size. However, the occurrence characteristics of the main elements in the screened steel slag are unclear, and the impact of screening on steel slag performance is also unclear. This leads to a low utilization rate of steel slag, making it difficult for the downstream building materials industry to utilize it. To address this problem, this study took hot smoldering steel slag as the research object. Steel slag was classified by particle size, the occurrence characteristics of elements such as calcium, magnesium, and iron were analyzed, and the basic properties of steel slag were systematically evaluated. This work is intended to assist in optimizing the steel slag pretreatment process, and establish the performance evaluation method, quality control indicators, and application scheme for steel slag. The results show that there is a strong correlation between steel slag activity and chemical composition. Magnesium, aluminum, and silicon in steel slag are mainly enriched in fine particles （0-5 mm）, while calcium and iron are concentrated in coarser particles （>5 mm）. The early-age activity of steel slag powder is related to the high-alkalinity substances MgO and Al₂O₃ in the system,and the long-term strength development mainly depends on cementitious active minerals such as C₃S, C₂S, and C₂F in steel slag particles, which has a linear correlation with the alkalinity coefficient. The hydration activity, particle effect of steel slag, and its inhibition on cement hydration jointly determine the microstructure of the hardened paste, which in turn affects its macro strength. When the specific surface area is 420-470 m²/kg, the activity of hot smoldering steel slag is optimally exerted. The expansion of hot smoldering steel slag powder is a long-term continuous process. This study proposes a process route for improving steel slag stability, which can reduce cracking problems and soundness hazards caused by the late expansion of f-CaO, f-MgO, and periclase. This research provides a scientific basis and guidance for steel enterprises to effectively manage and realize the resource utilization of steel slag.