Abstract: Steel slag is the main solid waste in the steelmaking process. Its comprehensive utilization rate stands only at 30% (China, 2023), resulting in a large amount of resource waste and serious damage to the ecological environment. The optimal utilization of steel slag lies in reusing it in the steelmaking process. However, this process does not have any suitable phosphorus outflow pathway, resulting in continuous phosphorus enrichment throughout the production process, hence hindering the efficient utilization of steel slag. Phosphorus, a nonrenewable resource vital to agriculture and industry, is a suitable secondary resource in steel slag. Phosphorus obtained upon separation from steel slag can be utilized in the phosphorus chemical industry, and the tailings can be reused in metallurgical plants. This study reviews the following: (1) sources and properties (physical and chemical properties) of steel slag; (2) current status of steel slag treatment and utilization at home and abroad; (3) phosphorus enrichment and separation technology in steel slag. Moreover, the economics of phosphorus recovery from steel slag is described, and a technology for full utilization of steel slag in hot-state recycling is proposed. The following conclusions are drawn: (1) China’s steel slag production and stock are massive. However, the existing technologies and methods are insufficient for the effective handling and utilization of such a considerable quantity of slag. Consequently, the experience of foreign developed countries in steel slag treatment cannot be entirely replicated in China. Accordingly, we need to devise a novel approach to steel slag treatment that aligns with China’s laws, policies, and standards associated with steel management. (2) The current mainstream steel slag treatment technology is unable to effectively address the challenges posed by phosphorus enrichment under harsh conditions, suffering limited phosphorous removal efficiency. In addition, the lack of a slag utilization pathway hinders the full-scale promotion of this technology. The steel slag gradient utilization model, despite its potential, requires further technical optimization to overcome the aforementioned challenges. (3) The carbon thermal reduction method has been demonstrated to be effective in the extraction of phosphorus from steel slag, with a high degree of removal efficiency. The comprehensive utilization rate of steel slag can be increased by steel enterprises so as to reduce environmental pollution and resource waste. The steel slag treatment, steel slag reduction, phosphorus extraction, and tailing slag recycling processes can be employed to efficiently treat and use steel slag, thereby enhancing both the steelmaking process and steel slag reduction efficiency. Moreover, the tailings can be returned to the converter for reuse as steelmaking or sintering flux, thereby fully utilizing the sensible heat present in the slag. Furthermore, this approach offers considerable promise for future applications. (4) This research group proposes a full recycling technology for steel slag based on carbon thermal reduction-based dephosphorization. During the pouring process of hot steel slag, phosphorus present in it is separated in a proper manner and ensuring suitable process conditions can result in a dephosphorization rate of 79.25%. Application of the new steel slag treatment process and hot-state full utilization technology is expected to facilitate the greening of the metallurgical industry.