Abstract: On the basis of observation and description of cores, combining with the logging and seismic reflection characteristics, the complex conglomerates of the Baikouquan formation in Mahu sag of Junggar basin were studies. The Baikouquan conglomerates is proximal coarse fan delta genesis and can be subdivided into fan delta plain,fan delta front and pro-fan delta, according to the sedimentary characteristics of the conglomerates, and the front can be subdivided into inner front and outer front. In view of the sedimentary structure, particle shape, arrangement and support forms and cementation type, etc, the nine kinds of conglomerate lithofacies and the six lithofacies associations were described, which reflect different sedimentary microfacies environments, respectively: debris channel, braided channel, braided distributary channel, braided channel bar, underwater distributary channel and lobate body. The color, grain size, particle contact relationship, sedimentary structure, compositional and textural maturity of the conglomerates were compared among Xiazijie, Huangyangquan and Karamay fan deltas. And then the control factors, developed positions, source of supply and gradient of slope of the development of the fan delta were cleared. Finally, the origin models for the proximal coarse fan deltas were established: Xiazijie fan delta is a long axis and brush-like one controlled by continual supply of debris flow; Huangyangquan fan delta is a lobate one controlled by intermont enough supply of debris flow and tractive current, and Karamay fan delta is a fan?like one controlled by piedmont partial supply of tractive current