The North Qilian Belt is located in the Center orogenic belt of China,which has the characteristics of typical orogenic belt.The sedimentary environment of the Silurian Angzanggou Formation is controversial,which leads to a great difference in the understanding of paleogeographic pattern in the northern Qilian area.The succession of Silurian Angzanggou Formation from the well-developed Laohugou Profile in Sunan area was measured,and the sedimentary filling sequence,sedimentary structure and the sedimentary facies types of Angzanggou Formation in Laohugou profile were analyzed in detail by outcrop observation,thin section identification and particle size analysis of clastic rocks.It is considered that Silurian Angzanggou Formation in the Laohugou profile of Sunan area,the western North Qilian area develops a large set of conglomerate,glutenite and pegmatite sandstones,the sedimentary structures are dominated by large trough cross-bedding,plate cross-bedding, parallel bedding and scouring structures,and the composition and structural maturity of sandstone fragments are relatively low.The content of rolling components in sandstone member is high.From bottom to top,the particle size becomes finer,indicating the distance from the source area becoming further.In conclusion,the sedimentary facies of Silurian Angzanggou Formation in Laohugou section of North Qilian Sunan area are braided fluvial facies,which indicates that in the early Silurian the mid-Qilian block has already collision with the Alax block.Meanwhile,the eastern part of North Qilian belt has not yet collided.Thus,it can be inferred that the collision of North Qilian in the early Silurian was an irregular point collision,happening in the western part of the orogen earlier than eastern part.This will directly affect the understanding of the Silurian paleogeographic pattern and tectonic setting of the North Qilian Orogenic belt.