The Maoniushan Formation in the northern margin of the Qaidam Basin consists mainly of purple red terrestrial sand and conglomerate, but its depositional age has long been controversial. This study has carried out an investigation for the depositional characteristics, paleocurrent direction and composition of gravels and LA- ICP- MS dating. The results show that the Maoniushan Formation is characterized by alluvial deposit, with the main paleocurrent directions from northwest to southeast and from southeast to northwest. The composition of gravels is quite distinct in two profiles, with the Chengqianggou profile dominated by carbonate and the Wanggaxiu profile being quite complicated. The youngest U- Pb ages of detrital zircons in sandy fillings of conglomerate can be divided into three populations: 360~560 Ma, 890~1050 Ma and 220~2500 Ma. They represent three tectonic events, including ocean- continent subduction to continent- continent collision of the North Qaidam in the Paleozoic, Grenville collisional orogeny in the Neoproterozoic and continent growth in the Late Archean. The youngest U- Pb age of detrital zircon is 365±3 Ma. Combined with regional background and paleontological data, we suggest that the depositional age of the Maoniushan Formation is Late Devonian. The U- Pb ages of the third and forth granitoids are consistent with the youngest U- Pb age of our detrital zircons, indicating the two episodes of granites have undergone rapid exhumation. Combined with previous studies, we suggest that the North Qaidam had evolved into the post- collision stage during the deposition of the Maoniushan Formation.