Abstract: The Hexi area is located at the tectonic convergence of the north Qilian Caledonian orogen,Alex block and Ordos block where was the transitional period between the early Paleozoic oceanic stage and the Mesozoic continental basin developing stage in the mid-late Paleozoic age.The explanations proposed for the formation mechanism of the mid-late Paleozoic basins are varied.During the exploration work in this area the authors discovered syndepositional faults in the late Devonian and early Carboniferous strata and researched their contact relations.On the basis of the field works combining with previous studies,the authors discussed the characters and formation mechanisms of the mid-late Paleozoic basins in Hexi area.The results indicated that the basins are neither foreland basins nor impactogen,but extensional superimposed basins caused by the detachment of the north Qilian Caledonian orogenic belt,and the basin formation would be simultaneously with the stage of the Paleo-tethys.