Abstract: By dissecting the evolution characteristics of the proximal-distal drainage systems of the modern alluvial fan in the piedmont of the southern Tianshan Mountains, the relationship between the sandy conglomeratic sediment variation and the transport distance, the structural characteristics, the slope gradient of fan surface, and the controlling effect of tectonic activities on sediments, it is considered that there are many new types of alluvial fans developed in the piedmont of the southern Tianshan Mountains, which are influenced by both drainage system and structure. The drainage system on the surface of the alluvial fans is distributed from provenance area to alluvial plain, which has the evolution features of intermountain river-single braided channel-composite braided channel zone-low curvature single runoff channel-seasonal river. The sudden increase of the slope gradient on the alluvial fan surface caused the changes of river channel type, channel width and sediment grain size. Meanwhile, a "steep in the north and gentle in the south" fold belt which is parallel to the provenance area in the southern Tianshan Mountains and extends tens of kilometers formed in the early deposited alluvial fan of weak consolidation diagenesis under the action of tectonic uplift. Like the secondary step in the piedmont area, the topographic slope of the depositional area increased and the sandy conglomerate with weak consolidation diagenesis provided the secondary source for the present deposition. Under the combined action of active drainage system and fold belt resulted from tectonic uplift, multi-stage sandy conglomeratic alluvial fan occurred. Based on the above understanding, the paper establishes a new model of alluvial fan sedimentary evolution under the combination of drainage system and tectonism in arid-semi-arid climate, which provides references for the genetic mechanism of large-scale distribution of sandy conglomeratic sediments in foreland thrust belt