Through the structure, composition and occurrence of sillimanite, the formation process of sillimanite has been discussed and summarized in this paper. In composition, the protolith of the sillimanite-bearing metamorphic rocks is not necessarily pelite rocks, the sillimanite-rich rocks does not necessarily correspond to any pelite rocks. The Al-rich feature of the pelite is favorable for occurrence of sillimanite, but sillimanite may not occur in the rock even under appropriate metamorphic temperature and pressure condition. The differential migration of components is essential for sillimanite. The formation of sillimanite is closely related to deformation and metamorphism, especially anatexis in metamorphic rocks. Leucosome, pegmatite and granite all can evolve through metamorphism and anatexis. The key to the formation of sillimanite is the migration and leaching of alkaline(-earth) metals and their separation from sial components, resulting in the relative enrichment of Al₂O₃. Sillimanite, especially fibrolite, corresponds to the early differential deformation-metamorphism stage. In the process of component migration, the flow of solution or melt components leads to the false image of "stress orientation" of sillimanite, the aggregation of sillimanite and related minerals, and multiple evolution stages of different metamorphic minerals. Although different variants of Al₂SiO₅ have the same composition, their rock types or environments are still significantly different. On the one hand, their formation is not only controlled by temperature and pressure conditions, but also the acidity or alkalinity of the fluid or melt, which is critial in affecting the coordination state of aluminum.