The resource utilization of magnesium slag is an urgent problem. This paper uses magnesium slag and modified magnesium slag as the main cementing materials, and magnesium slag based cementing materials (UCGB) and modified magnesium slag based cementing materials (MCGB) are prepared by adding fly ash. The flow characteristics, mechanical properties, microstructure and hydration characteristics of the filling materials are compared and analyzed. The results show that when the mixing ratio of modified magnesium slag and fly ash is 4/1, the prepared MCGB sample has excellent mechanical properties, and the uniaxial compressive strength is up to 4.213 MPa after curing for 28 days. With the growth of curing age, MCGB sample hydration produces a large number of hydration products such as C-S-H gel, Ca(OH)2 crystal and filamentous Ettringite, which are interwoven and agglomerated with other silicate oxides (［Fe, Mg, Al］2.5［Si, Al］2O5［OH］4). Filling in the pores and holes inside the sample is helpful in improving the mechanical properties and durability of the MCGB sample. Compared with the MCGB sample, the mechanical properties of the UCGB sample are not ideal. The early strength of the UCGB sample is low, and only a small amount of Ettringite and Ca(OH)2 crystals are produced by hydration, forming a porous microstructure. In addition, the infrared spectrum curve analysis shows that the characteristic frequency of β-C2S appears near 997 cm-1 of the modified magnesium slag, while the characteristic identification spectrum band of γ-C2S appears near 820 cm-1 of the magnesium slag. Combined with the X-ray spectrum analysis, the mineral phase of the modified magnesium slag mainly changes to β-C2S. However, the mineral phase of the original magnesium slag is mainly γ-C2S, which has almost no hydration activity, so it is unsuitable for mining as a cementitious material. Therefore, this study aims to provide scientific basis and guidance for the preparation of new mine cementitious materials based on modified magnesium slag.