Abstract: Hundreds of millions of OOIP discovered in the area around Mahu sag, Junggar basin reveals the giant oil and gas potential in the area. Exploration practice shows that the sedimentary system in the area around Mahu sag which is different from that of alluvial fan previously found is a set of shallow-water, gentle-slope fan delta system. To deeply understand the depositional characteristics and formation mechanism of this system and based on flume simulation device in Yangtze University, simulation and experiments are carried out for the formation process of the sedimentary system. In the experiment, 3 levels of slope-break and 3 periods of lake level change are set, which correspond to 3 depositional periods of T1b1, T1b2 and T1b3, respectively, and gravity flow and tractive current are used to simulate the process of fan body formation. The modeling result shows that a kind of retrograding sequence was formed due to the rise of lake level, resulting in obvious overlapping of sequences. The investigation on reservoir internal structures indicates that laminated interactive deposits of gravity flow and river are more commonly found in the fan-delta plain, and the deposits of tractive current and secondary gravity flow resulted from progradation occur in the fan delta front. The boundaries of fan bodies caused by gravity flow and progradation in different periods are not clear and it is difficult to distinguish them. Lake level, slope-break, sedimentary energy(elevation of ancient landform) and the compositions of deposits have significant influences on fan development during the deposition process