Abstract: With the rapid development of the global magnesium industry, a large amount of magnesium slag is continuously accumulated in the process of magnesium smelting based on the Pidgeon process, and the threat to the environment is increasing. How to treat the magnesium slag in a large-scale harmless manner and realize the resource utilization of the magnesium slag is very important to promote the development of the magnesium industry and the protection of environment. By summarizing the basic characteristics and application status of magnesium slag, the mechanism of expansion and gelation characteristics of magnesium slag was analyzed. In view of the low activity of magnesium slag, most of the applications of magnesium slag are often combined with activation methods such as grinding, quenching and adding activators, but the problem of low activity and expansion of magnesium slag is still not fundamentally solved. Therefore, this paper proposed the use of chemical stabilizers to modify magnesium slag, clarified the modification principle and modification process, and successfully obtained a modified magnesium slag(MMS) through indoor and industrial experiments. The MMS is in the form of complete pellets, has high hardness and stable properties, is rich in β-Ca2SiO4, and has good hydration activity. On this basis, a new type of paste filling material(MFPB) was prepared using the MMS mixed fly ash as a gelling agent and aeolian sand as aggregate, which has some good basic properties. Among them, the mini-slump value of the MFPB is between 127-141 mm, the 28 d strength can reach 5.249-7.491 MPa, and it has good environmental stability. Based on the above theoretical and experimental research, some industrial tests were carried out in Mahuangliang Coal Mine, and the MFPB was successfully used for underground filling, and the average strength of 28 d underground coring can reach 6.23 MPa. If the technology is promoted, it is not only enable to carry out large-scale industrial treatment of solid wastes such as magnesium slag and fly ash, but also greatly reduces the cost of mine filling.