Granitic pegmatite Li deposits account for approximately half of global Li production, while the Li mineralization mechanism remains enigmatic. Here we present Li isotope data on the albite from 8 textural zones of the well-characterized Koktokay No. 3 Li-Be-Nb-Ta-Rb-Cs mineralized pegmatite in Xinjiang, China. Our results reveal that there coexists two important processes during the internal evolution of the pegmatite: (1) fluid exsolution and (2) Li unloading. The former facilitates Li enrichment and the later results in spodumene crystallization. These “fluid transportation” processes can be indicated by a negative correlation between δ7Li value and Li content. Globally, the statistical geochemical data of Li mineralized pegmatites have also yielded a negative linear correlation between δ7Li value and Li content in both of whole rock and separated mineral samples, providing compelling evidence that “fluid transportation” is the key mechanism responsible for Li mineralization in granitic pegmatites. This finding is of great significance for Li metal deposit modeling, highlighting an urgent need to better understand the dynamic processes of magmatic-hydrothermal interaction during the crystallization of pegmatite magma.