Abstract: Gangtangcuo granite widely distributes in Shuanghujiaomuri area, Tibet and it is a set of Late Triassic felsic intrusive rock. In 2008, during regional 1∶50000 geological mapping, we found that the coarse-grained phenocryst in biotite adamellite belongs to annulus stripe K-feldspar. In this paper,we described the mineralogy, petrology characteristics of the giant K-feldspar phenocryst. By using electronic microprobe method, a detailed analysis of the component variation from the edge to the core within this phenocryst was also conducted out, and through two-feldspar geologic thermometer, crystallization temperature of phenocryst was also calculated. Finally, based on the chemical composition changes of the phenocryst and crystallization temperature, combined with the petrology and geology characteristics, we made a primary conclusion that the granite was formed because of magma mixing.