Abstract: In this paper,the formation mechanism and regional occurrence of the unconformity on the top of Precarboniferous in Tarim Basin were studied,and then,its constraints on the migration and accumulation of oil/gas,and roles in the preservation or destruction of pools were disccused. As the result,fault-flod umconformities,flod unconformities and onlay unconformities are that which significantly controlled oil/gas behaviour. The unconformity onthe top of Precarboniferous,together with the permeable zones immediately adjoining it,could act as conduits for oil/gas migration the scondary reserviors by dissolution and the onlay sandstone over the uncoformity could be the sites of oil/gas accumulation.