Ordovician is an important period of the Early Paleozoic collisional orogenesis in northern margin of Qaidam ba-sin.Arguments on the initial timing of continent-arc collision and the basin type､paleostructure-paleogeographic pattern last-ed till now.In this paper,on basis of the field observations and lab analysises on Early Ordovician clastic deposition in O-ulongbuluke block,combined with major elements､trace elements and rare earth elements measurements and analysises on 30 collected cubic samples.Results indicate that,clastic rocks from Shihuigou Formation has sedimentary and petrology cha-racteristics of rapid accumulation and low maturation of texture and component;the series of clastic rocks deposited in retr-oarc foreland under the background of active continental margin,detritus derived from the neighbouring upwelling basement composed by upper continental crust and island arc in the south;continental-arc collison caused by Qaidam block and Ta-njianshan island arc has begun since the Early Ordovician(488-472Ma),but the initial timing of continent-arc collision is no earlier than 493Ma.On the above mentioned basis,combining previous research results,we suppose that the Oulongbuluke block located to the north of Tanjianshan island arc during Early Paleozoic Era,basin subsidence and sedimentary evolution was controlled by oceanic crust subduction and continental-arc collision,retroarc stretch basin developed during Cambrian period,then transferred to retroarc extrusion foreland basin in the Early Ordovician,retroarc stretch turned to retroarc extrus-ion､sedimentary system transformation occurred approximately 490-480Ma.The conclusions presented in this study provide new sedimentary constraints on the initial timing of continent-arc collision.