Abstract: Using the data of dense well net in the oil development area, the features of the high resolution sequence stratigraphy of alluvial fan and the distribution patterns of sandbodies in different base-level cycles of Lower Karamay Formation (Middle Triassic) in the Karamay Oilfield of Junggar basin are studied. The results show that the hierarchy base-level ascending hemicycles are well developed, including 3 middle-term cycles, 7 short-term cycles and several super short-term cycles. It indicates the hierarchy change of the accommodation space to sediment supply ratios (A/S) in a vertical direction. Several smaller cycles with increasing A/S ratios are observed on a general increasing A/S background. The distribution patterns of sandbodies changed from sheet pattern at the bottom, through broad band pattern in the middle, to the narrow band pattern on the top of the Lower Karamay Formation. With the increasing A/S ratios which control the migration of braid channels, the width and thickness of sand bodies decreased.