Skarn deposits, as one of the most economically viable and highly studied deposits, is famous and commonly identified by its mineral zone, metal mineralization, and ore- forming fluid evolution. However, determining the diagenesis age of skarn is a challenge for the mineralization study of skarn deposit in the past decades. The Sangri copper deposit is a newly discovered porphyry- skarn deposit in the middle of the Bangong Co- Nujiang metallogenic belt, located in the Qinghai- Tibet Plateau. Chalcopyrite, pyrite, and molybdenite are the common metal minerals, occurring as veinlet, massive, and disseminated in the porphyry and skarn. The U- Pb age of garnet in the skarn dated by femtosecond laser erosion system with high- resolution inductively coupled plasma mass spectrometer is 83. 2±2. 3 Ma (MSWD=1. 2, n =26), from the Sangri copper deposit in the Bangion county, Tibet. These data showed that diagenesis age of skarn was late Cretaceous. At the same time, compared with the U- Pb age of zircon from the ore- bearing porphyry and the Re- Os age of molybdenite, U- Pb age of garnet was consistent with them within the error range, indicating that they were the products of the same mineralization event. Therefore, the accurate U- Pb dating of garnet can effectively confirm the diagenesis age of skarn deposit or porphyry- skarn deposit. It is a new geochronology method and provides useful support for the metallogenic mechanism study of skarn deposits.