Ordos basin is the first 100 million ton tight oil bearing basin in China. The Chang 7 member of the Triassic Yanchang Formation is a typical representative of the tight oil in this basin. However, there is limited data on the tight oil accumulation mechanism in the Chang 7 member.The tight oil accumulation mechanism of the Chang 7 member is comprehensively studied in order to clarify the reservoir forming mechanism of tight oil in the Chang 7 member, through the analysis of tight reservoir characteristics and oil migration and accumulation dynamics, combined with reservoir diagenetic evolution, physical property evolution and its corresponding relationship with hydrocarbon generation evolutionand reservoir wettability analysis.The results show that high- quality source rock’s hydrocarbon generation pressurization and the residual pressure of mudstone undercompaction provide power for tight oil migration. The tight oil in Chang 7 member shows two- stage filling, corresponding to the middle- late period of Late Jurassic and middle- late period of Early Cretaceous. During the first stage of tight oil filling, the reservoir has not been fully compacted, and the dissolution occurred before the large- scale filling of the second stage, which improved the physical properties of the tight reservoir. The physical properties of the reservoir in the two stages of oil and gas filling and accumulation were not as tight as they are today, and the relatively good reservoir conditions provided good migration and reservoir conditions for tight oil. The tight reservoir mainly consists of neutral and lipophilic reservoirs, which reduces the resistance to oil and gas filling. The high source reservoir pressure difference and high- temperature and high- pressure formation conditions allow low- viscosity crude oil to enter tight reservoirs and form reservoirs. This result is of great significance for the next tight oil exploration and development in the Ordos basin.