The Oligocene granitic rocks in the Sangri area are mainly composed of quartz monzodiorite, monzogranite and syenogranite. The weighted average ages are 31.2±0.5 Ma, 31.0±0.5 Ma and 29.0±0.1 Ma, respectively, and the emplacement age is the Oligocene. The rocks are developed abundant melanocratic enclaves of different shapes. The K₂O content is 3.24%~5.73%, belonging to the high potassium calc-alkaline rock, with the characteristics of metaluminous to peraluminous rock evolution (A/CNK: 0.90~1.02, 0.98~1.07, 1.10~1.29). The rocks are LREE enriched type and the europium depletion shows an evolutionary characteristic from weak to strong (Eu/Eu^*: 0.66~0.75, 0.40~0.55, 0.04~0.09). They are relatively enriched in LILEs such as Rb, HFSEs such as Th, U, and LREEs such as (La, Ce) Nd, Sm, but depleted in LILEs such as Ba, Sr and HFSEs such as Nb, P, Ti (Ta). The fitting curves formed by SiO₂ vs. other major elements, rare earth elements and trace elements are highly coupled and the R² values are all greater than 0.90, which indicated they were products of the evolution of the same magma at different stages. The average zircon saturation temperature (t_Zr) is about 700℃ which may represent the initial temperature of melt formation. Based on the analysis of enclaves, structural styles, petrogeochemical characteristics and Hf isotopes, etc., the petrogenetic type of Oligocene granites in the Sangri area belongs to I-type granite. They were formed in the tectonic environment of rapid uplift due to the intracontinental collision of the Indian and Asian continents. The formation mechanism was the delamination or convective removal of the Indian continental slab during low-angle subduction. A small amount of fluid was released during the slab sinking process which caused the partial melted mantle-derived magma underplating and induced lower crust melting of the Lhasa terrane to form mix-derived magma. At the same time, the magma emplaced along the northeast-trending extensional fault in the Sangri area, eventually further leading to the continental crust accretion of the Tibetan Plateau. It enriches the evolutionary history of multi-phase accretion of the Qinghai-Tibet Plateau continental crust.