The pore structure and fractal characteristics of tight shale oil reservoirs were comprehensively studied by means of X-ray diffraction, casting thin section, high pressure mercury injection and nuclear magnetic resonance, based on the core of Chang 7 member of Yanchang Formation of YP well in Zhidan Oilfield, Ordos Basin. The results show that the reservoir is dominated by lithic feldspar sandstone, and the felsic minerals are dominant. The whole reservoir is characterized by low porosity ( 3.94 % -11.64 % ) and low permeability ( 0.106-0.334x10-3μm2). The pore type is dominated by primary intergranular pores, and superimposed dissolution pores are developed. The median radius of pore throat is about 0.05 μm, and the overall connectivity is limited. The NMR results show that the T2 spectrum is dominated by short relaxation time, the movable fluid saturation is generally low ( 8.91 % ~ 43.98 % ), and the reservoir is dominated by bound fluid. The fractal dimension is 2.39 ~ 2.57, reflecting that the pore structure is complex and the heterogeneity is strong. The fractal dimension is negatively correlated with pore throat scale and permeability, and positively correlated with clay content, but weakly correlated with single pore throat parameters, indicating that it essentially reflects the overall complexity of pore structure rather than local geometric characteristics. It is believed that the evolution of pore structure is controlled by the dual effects of " cementation densification-dissolution transformation’, in which dissolution is the key factor to improve pore throat connectivity and fluid mobility. Fractal dimension can be used as a comprehensive evaluation parameter to connect pore structure and seepage capacity.