While promoting economic and social development of China, open-pit coal mining will also cause irreversible harm to the surrounding environment due to disturbing the topsoil. Therefore, ecological restoration measures are urgently required. Vegetation cover is an important part of mine ecological restoration work. Suitable soil structure and nutrient conditions are the basis of normal plant growth and development. In this study, the slag, sand, humic acid, diammonium phosphate (DAP) and urea were selected as the improved materials to adjust the structure and fertility of the slag and improve the slag into a suitable vegetative substrate for plant growth. From among various configurations of the vegetative substrate, the most suitable substrate for plant growth was filtered by comparing the physical and chemical properties and their effects on Alfalfa growth under different treatments. The results showed that the application of fertilizers could increase the nutrient content of the vegetative substrate and increase the pH, but the growth of Alfalfa was not optimal under the high fertilization treatment. The addition of sand could mitigate the structural problems of slag, and sand also showed a certain degree of regulation on fertility. Moreover, the addition of appropriate amounts of humic acid can enhance slag maturation and stabilize the chemical properties of the vegetative substrate. According to the results, the most optimal slag-sand-humic acid ratio for the vegetative substrate of the dump site of the Dafeng Open-pit Coal Mine is 18:1:1. The findings are expected to offer a reference for the improvement method of slag in this area, which alleviates the shortage of soil resources. It provides support for ecological environmental protection in soil source area, and also offers a new idea for the resource utilization of slag in the ecological restoration.