Abstract: The Ordovician reef?flat complex is one of major oil?gas bodies in carbonate reservoirs in Tarim basin. The occurrence, reef building organisms and sedimentary facies changes of the complex were probed by analyzing characteristics of the ancient geographical setting, petrology, sequence, sedimentary facies, paleontology and geochemistry It is found that the complex is mainly developed in platform marginal facies of member?2 of Yijianfang formation of the middle Ordovician; the reef building organisms are dominated by the tray sort,followed by the sponge. δ13C and δ18O values fluctuations mean this sedimentary stage of Yijianfang formation belongs to sequence from ingression to regression with the relatively stable sea?level changes