In the southern margin of Junggar Basin, multiple rows of thrust faults and anticlines are widely developed under the tectonic stress during the Himalayan period, and three reservoir-seal assemblages including the upper, middle and lower are developed vertically, among which the lower assemblage with multiple sets of Jurassic-Cretaceous reservoir is the most important successor for oil and gas exploration. The Lower Cretaceous Qingshuihe Formation has two provenance systems from south and north respectively, and the braid-delta sandbodies in the north provenance system is widely distributed. High-production gas flow was obtained in Qingshuihe Formation of Well HT1, which not only indicates the great exploration potential, but also confirms a set of high-quality ultra-deep reservoir (the depth is more than 7 000 m). Based on the regional geological data, this paper analyzes the rock minerals, physical property and pore structure of the sandstone in Qingshuihe Formation in detail by means of thin section observation, pore permeability analysis, mercury injection analysis, field emission scanning electron microscope observation, laser confocal microscopy, and other analysis and testing methods. The diagenetic evolution sequence of the reservoir is established by the former result combined with burial history. The studies show that the reservoir rock type of Qingshuihe Formation in the middle part of the southern margin is mainly feldspathic litharenite sandstone with high texture maturity, which has moderate cement content. The reservoir space is dominated by primary pores and fractures. The reservoir is in middle diagenetic stage B. Based on the comprehensive analysis, the formation of relatively high-quality reservoir of Qingshuihe Formation is controlled by four factors: (1) sandstone with high texture maturity from distal provenance is the foundation to form high-quality reservoir, (2) thermal compaction effect in sandstone is relieved by low geothermal gradient, (3) overpressure controlled by fluid expansion reduces the loss of pore-space during compaction, (4) fractures improve the seepage capability of the sandstone reservoir. This research provides an important geological example for the exploration of ultra-deep clastic reservoirs in continental basins.