Abstract: There is Tetragraptus approximatus, the typical graptolite fossil cluster which distribute in the bottom part of Arenigian of Early Ordovician in Xainza area, Gangdese orogenic belt. In situ zircon 206Pb/238U LA-ICP-MS dating on the rhyolite magmatic genetic zircons yielded a weighted average age of the (500.8±2.1)Ma; this has confirmed the existence of Cambrian sediments in the mid-west part of Gangdese orogenic belt for the first time. It′s indicated by the primary composition analysis that it is formed in shallow marine deposition and active tectonic setting, it also shows that the existence of Cambrian strata are not only local in Xainza County, but also in larger area.