Abstract: The Yanchang Formation Chang 8 reservoir in the Jiyuan region of Ordos Basin is rich in tight oil resources. Quantitative characterization analysis of the present-day in-situ stress field is a key component to ensure profitable development. This study uses methods of imaging logging interpretation, in-situ stress experimental testing and conventional logging calculations to conduct a comprehensive study on the present-day dominant orientation, vertical variation patterns and mechanisms of in-situ stress in the Chang 8 reservoir. The results show that: (1) The horizontal maximum principal stress in the Yanchang Formation Chang 8 reservoir of Jiyuan region is mainly in the near E-W direction; (2) The horizontal maximum and minimum principal stress magnitude of the Chang 8 reservoir is in the range of 49.17~65.68 MPa and 41.87~56.31 MPa respectively. The vertical principal stress value is the largest, which belongs to the normal fault type in-situ stress mechanism. The horizontal principal stress difference is between 2.78~13.39 MPa, which inidcates the in-situ stress conditions to form a complex fracture network; (3) Factors such as differences in lithology and rock mechanical properties, burial depth, and discontinuous structural planes are the main controlling factors affecting the vertical changes and directions of present-day in-situ stresses; (4) Vertically, hydraulic fractures are easy to extend upward and downward, and they will preferentially extend along the direction of the horizontal maximum principal stress laterally. The research results will help provide scientific basis and geological reference for further exploration and development of tight oil in the study area.