The Sonid Left Banner area, located in the southeast of the Central Asian Orogenic Belt and north of the Suolun suture zone, develops a characteristic volcanic-sedimentary rock series of the Lower Devonian Obootinghundi Formation. The strata are mainly composed of andesite, tuffaceous sandstone, sandstone and siltstone as well as pyroclastic rock. The rock has poor roundness and shows near-source sedimentary characteristics. The results of geochemical analysis show that the sandstone has medium SiO₂ and K₂O content, high Na₂O content, and low Fe₂O₃ and MgO content. Trace elements show right-inclined feature on the spider map, and the content and change trend are similar to those of the upper crust; rare earth element distribution characteristics display LREE-enrichment but HREE-depletion. The average (La/Yb)_N is 6.44, and the Eu anomaly is obvious (Eu/Eu^* average 0.72). Combined with regional analysis, the authors hold that the sandstone was formed on the active continental margin and the continental island arc. The source rocks are mainly metamite with minor andesite, and it is inferred that the Early Devonian paleo-Asian ocean was still in a tectonic setting of northward subduction. The source of the Obootinghundi Formation should mainly be the upper crust felsic rock series of the Uliastai area in the north.