Abstract: This study was based on field study on Baiyanghe and Manas alluvial fans in North Tianshan Mountain. Braided belt is the main sedimentary area on the surface of arid braided alluvial fan. Sediments were unloaded in low potential area and sedimentary area laterally migrated along with the low potential area migrated. Braided channels show as string on the surface of alluvial fan and distributed in a wider area in the margin of fan. Gravel sediments in the braided belt have good porosity and permeability. Sedimentary characteristics and scale of alluvial fans are controlled by tectonic activities, climatic conditions, provenance supplies and river system. Braided alluvial fans are main ancient alluvial fans types in the northwest Junggar Basin and they occurred in the basin margin, showing more sedimentary characteristics of braided river.

Key words: North Tianshan Mountain, braided alluvial fan, Junggar Basin, ancient alluvial fans