Abstract: The upper Wuerhe formation of Permian and Triassic Baikouquan formation in Mahu sag and its adjacent areas are a set of near?source coarse deposits under the background of depression basin that are large?scaled shallow retrogradation fan?delta deposits under the background of gentle slope, and 13 lithofacies and 7 sedimentary microfacies are identified based on core observation. Multiple fan deltas are developed on the slope of the sag, and the fan bodies connect with each other. The formation of this kind of large?scaled shallow retrogradation fan delta is closely related to 3 geological conditions. The first is that continuously uplifting periphery ancient mountains and stable river systems provided sufficient provenance for each fan during deposition; the second is that there was a good paleogeographic background such as large basin, shallow water and gentle slope; the third is that continuous lake transgression and multi?stage slope?break are significant conditions for large?sized superposition and merging of sand bodies. A sedimentary model of large?scaled shallow retrogradation fan delta is established, which provides guidance for petroleum prospecting and reserves estimation in Mahu sag