Poor soundness remains a critical barrier to the large-scale utilization of steel slag in construction materials. This review systematically examines the origins of soundness problems, focusing on expansive components such as free lime (f-CaO), free magnesia (f-MgO), RO phase, and sulfur-bearing compounds. Their physicochemical properties and mutual interactions are discussed in relation to their role in delayed hydration and volumetric instability. The paper then reviews mainstream soundness evaluation methods, including f-CaO/f-MgO content analysis, immersion expansion ratio, autoclave chalked ratio, autoclave expansion ratio, and the boiling method. A comprehensive comparison of national and industry standards is provided, highlighting the diversity of test requirements for different applications. Finally, key treatment strategies for improving the soundness of steel slag are critically assessed. Among them, the hot braising process and high-temperature reconstruction are recommended for early-stage control in steel plants. Other methods such as stockpiling, acidification, autoclave pretreatment, grinding, and carbonation are discussed with respect to their effectiveness, limitations, and suitable application scenarios. The review aims to provide both theoretical insights and practical guidance for the safe and efficient use of steel slag in sustainable construction.