This paper aims to study the characteristics and distribution of tight sandstone reservoirs deposited by gravity flow in deep water. Specifically, the study focuses on the 7th Member, Yanchang Formation, Triassic (the Chang- 7 Member), in Heshui area, Ordos Basin. Methods: The analysis is conducted through the core description, cast thin slice, and scanning electron microscope observation. By combining physical property data and analytical laboratory data, the study analyzes the petrology, pore types, and diagenetic characteristics of the Chang- 7 Member reservoir in the study area. Additionally, the formation mechanism of a high- quality reservoir is discussed based on sedimentary and diagenetic characteristics. Results: The reservoirs of the Chang- 7 Member in Heshui area mainly comprise feldspathic lithic sandstone. The pore types are mainly solution and residual intergranular pores, which belong to the ultra- low porosity and ultra- low permeability reservoirs. The main diagenetic processes are compaction, cementation, and dissolution. The quality of gravity flow tight sandstone reservoirs is controlled by sedimentary facies, compaction resistance, illite mineral content, and feldspar and other minerals dissolution. Reservoirs deposited in restricted channels generally have the best reservoir quality under relatively weak compaction, cementation, and strong dissolution. Sandstones in different sedimentary environments have differences in particle size, sorting, and sedimentary thickness, which can control reservoir quality by influencing diagenetic type and intensity. Conclusions: There are differences in grain size, sorting and thickness of reservoirs formed in different sedimentary environments, which lead to differential diagenesis and affect reservoir quality. Based on this, a quality evolution model of tight sandstone reservoirs under gravity flow sedimentary background is established. The research results can provide a reference for predicting and evaluating the same type of reservoir.