Abstract: Based on field work in this study in combination with previous researches, the authors recognized a regional unconformity within the Quanji Group. This unconformity overlies the Hongzaoshan Formation. It is featured by the iron palaeo-weathering crust, interruption of sedimentation and erosion of the surface over and below the unconformity. There also exist remarkable differences of paleontological associations and depositional environment on both sides of the unconformity. The Hongzaoshan Formation beneath the unconformity represents carbonate platform sedimentary facies. The Heitupo, Hongtiegou and Zhoujieshan formations and Xiaogaolu Group overlying the unconformity show the sedimentary environments characterized by transform from glaciofluvial deposit to supralittoral-intertidal zone, suggesting a transgression process after the formation of the unconformity. This unconformity may be related to the extension tectonic activity of the late Neoproterozoic rift setting on the northern margin of the Qaidam Basin. This interpretation indicates that the contact relation between the Hongzaoshan Formation and the Heitupo Formation of the Quanji Group is not a conformable contact as previously believed. This unconformity formed in Cryogenian-Ediacaran period. The differences of paleontological associations and depositional environment on both sides of the unconformity may be related to the breakup of Rodinia supercontinent and snowball Earth deglaciation.