The Ke' eryin ore field, located in the Songpan- Ganzi orogenic belt, is one of the most important pegmatite- type lithium ore concentration areas in China. To further reveal the genetic relationship between granite and pegmatite, the evolution process of the Dangba pegmatite, and the enrichment mechanisms of rare metals, this paper selects the penetrating mineral- mica as the research object. Based on previous findings, detailed petrographic and mineralogical identification, electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA- ICP- MS) were carried out. By studying the textural characteristics and chemical composition of mica, a Rayleigh fractional crystallization model was established to simulate the magmatic evolution process. The results show that the Dangba pegmatite probably formed via extreme fractional crystallization of the Ke' eryin two- mica granite melt, which mainly experienced an evolution process from a magmatic to a hydrothermal stage. The primary muscovite crystallized in the initial magmatic stage, while the secondary muscovite formed during the magmatic- hydrothermal transition. The dominant substitution mechanisms in this process are 4AlTot3SiIV+□VI and 3LiVIAlVI+2□VI (□ represents a vacant site). From albite pegmatite to albite- spodumene pegmatite, the muscovite series evolves from muscovite → iron- lithium mica → lithium mica, with the K/Rb ratio decreasing and the contents of Li, Rb, Cs, and F increasing. This indicates that fractional crystallization is the main mechanism for the enrichment of rare metals in the Dangba pegmatite. In addition, the Dangba pegmatite shows an extreme degree of crystallization differentiation. Its melt is Li- rich and experienced relatively weak fluid activity, which was of great significance for the enrichment and preservation of lithium.