The Devonian of the North Qilian Mountains consists of the Middle—Lower Devonian Laojunshan Formation and the Upper Devonian Shaliushui Formation. The Laojunshan Formation was dispersed over the piedmont and intermountainous basin along the PaleoQilian orogenic belt and is composed of coarse clastic rocks. The detrital and chemical components can directly reflect the features of the provenance and its tectonic setting. The geochemical analyses of the clastic rocks of the Laojunshan Formation show that K2O/Na2O, Al2O3/SiO2, LREE/HREE, Eu/Eu*, (Gd/Yb)N and CIA are 0.13～8.13, 0.15～0.28, 3.49～8.71, 0.63～0.91, 1.13～1.56 and 50～87 respectively. All the chemical parameters are distributed widely with the analyses of the discriminatory plots of provenance. The results indicate that detrital rocks are mainly derived from the Qilian orogenic belt to the south and a little from the mainland debris to the north, which shows the dual sources characteristics of a foreland basin; in the The Middle—Lower Devonian, the sediments of the North Qilian Mountains represent the late molasse stage of the foreland basin.