Located within West Kunlun terrane on the southern margin of Caledonian Kudi-Subashi suture zone in Qinghai_Tibet plateau and formed from the end of Ordovician to the beginning of Devonian, Kudibei rock body consists of early monzonitic granite and late granite, which are composed of microcline, plagioclase, quartz and biotite. Biotite assumes anhedral interstitial form and was crystallized later than ligh_colored minerals. The accessory mineral zircon has perfect crystal faces of (110) and (111), suggesting relatively high crystallization temperature 215 (850~900℃). Chemical composition is characterized by rich K, Th, REE, Nb, Zr, Y and poor Ca, Mg, Ba, Sr and transitional elements. All these characteristics show that it belongs to A-type granite. Both Sr, Nd, O isotopes and Rb/Sr ratios demonstrate that the protoliths are oceanic crust surface abyssal sediments. On the basis of regional tectonic framework, combination of associated rocks and structural discrimination diagram, the authors analysed the formation environment of this rock body. The results show that the rock body was formed in a relatively stable extension environment of late orogeny and belongs to A-type granite. It was em- placed during the falling off of oceanic plate sheets and formed by earlier successive partial melting of oceanic crust sediments which subsided with the subduction of the oceanic plate. The researches indicate a mechanism for the recycle of crustal materials to the earth crust and are of great significance to the investigation of Early Paleozoic tectonic evolution of Qinghai_Tibet plateau and its geodynamic mechanism.