The approxim ate N ——S trending grabens formed in east——west extension are the m ost distinct structures of the Qinghai—Tibet Plateau．The study of grabens will provide important evidences for the Cenozoic evolution of the Qinghai—Tibet Plateau， so it’s fl hot spot of tectonics studies． Spatial distribution， ages， forming mechanism s and relationship with the uplifting of the plateau are key problems of graben study today．Base on the new field works in Shuanghu and Tuotuohe regions of central Tibet by us and the datum have been obtained by other researches on the grabens．W e proposed that，the graben structures are distributed in the m ost part of plateau；including Himalayan，Gangdese and Qiangtang block．The grabens were forming in 1 4～ 7 M a．And the N-S extensional structures of Tibet is the result of the Plateau rapid uplift as fl whole caused by deep underground dynam ics after the crust shortening deformation of the Plateau，but not the result of ravitational collapse that followed the plateau uplifting to the highest elevation．The formation of the grabens indicate that the plateau uplifting mechanism changed from early compressive stage to the late deep theremo—dynamic mechanical stage.