Reservoir pore structure is crucial to oil and gas exploration and development, but the characteristics of pore geometry, size distribution, and pore space distribution are difficult to describe with a single method. The microscopic pore structure of the Long 4+5 reservoir in the Chenghua area of the Ordos Basin was finely characterized by various methods, including core data, cast thin section data, scanning electron microscope data analysis, high pressure mercury pressure and fractal theory. The research results show that the Chang 4+5 reservoir in Chenghua area has an average porosity of 9.93% and a permeability of 0.41 mD, showing typical extra-low porosity and ultra-low permeability, complex pore throat structure, and strong inhomogeneity of the reservoir. The pore types in the reservoir are mainly intergranular pores, intra-grain solution pores, intergranular pores and microfractures; the pore throat radius of the reservoir is mainly distributed in the range of 0.06 μm-2 um; the pore structure of the reservoir in the study area is divided into three categories according to the mercury pressure curve. The study of the fractal characteristics of sandstone reservoirs by the mercury saturation method shows that the relatively small pores show good fractal characteristics with a mean fractal dimension of 2.31, while the mean fractal dimension of the relatively large pores is 4.90, indicating that the relatively large pores do not have fractal characteristics and have a complex pore structure with strong heterogeneity; the analysis suggests that diagenesis, fracture development and oversimplified pore morphology are the main reasons for the large fractal dimension of the relatively large pores in the study area The analysis suggests that diagenesis, fracture development and over-simplification of pore morphology are the main reasons for the large relative macropore fractal dimension in the study area. The calculated turning point radius has a good correlation with permeability and median radius, indicating that the turning point radius is suitable for characterizing the heterogeneity of pore structure, The results of the study have implications for the scientific exploration and development of hydrocarbon resources in the study area.