Taking melting-separated red mud as the research object, the change law of melt properties was explored in the process of melting-separated red mud slag fibrosis. The melting process and melting temperature of melting-separated red mud were studied by slag melting point and melting rate tester. FactSage thermodynamic software was used to simulate the mineral precipitation type, content and initial crystallization temperature during the cooling process of melting-separated red mud slag. X-ray diffractometer and field emission scanning electron microscope were used to study the mineral composition and micro-morphology of melting-separated red mud slag at different temperatures. The viscosity change of melting-separated red mud slag during the cooling process was studied by using the comprehensive tester of melt physical properties. The results show that the melting temperature of melting-separated red mud is 1 236 ℃. During the cooling process of melting-separated red mud slag, crystals begin to precipitate at 1 300 ℃. At first, magnesium aluminate spinel (MgAl2O4) is precipitated. In addition, after comprehensive analysis of melt properties of melting-separated red mud slag, it is clear that the temperature of preparing inorganic fiber by fiberizing melting-separated red mud slag should be higher than 1 433 ℃.