Characterizing pore structure is one of the most fundamental tasks in reservoir characterization; it is closely related to the calculation/interpretation of other critical parameters, such as permeability and capillary pressure. High-pressure mercury injection (HPMI), a low-pressure nitrogen gas adsorption (LP-N(2)GA), specific surface area (SSA) analysis, nuclear magnetic resonance (NMR), and fractal theory were used to study the pore structure characteristics and irreducible water saturation of tight reservoir samples from the Chang 7 formation in Ordos Basin, China. In this study, pores are mainly composed of mesopores and macropores. HPMI is more likely to detect macropores, while the distribution of mesopores is better characterized by LP-N(2)GA and SSA analysis. The capillary curves obtained by HPMI experiments are divided into two categories. The adsorption-desorption isotherms are divided into two groups, according to the rate of change of the desorption curve when the relative pressure is 0.5. The permeability contribution rate of different pore radius was studied through different methods, and the results showed the combination of HPMI and LP-N(2)GA can describe the microscopic pore structure of a reservoir more comprehensively than either method alone. The irreducible water saturation obtained by NMR test is greater than the irreducible water saturation obtained by HPMI. An irreducible water saturation model was established based on fractal theory and the capillary bundle model. The factors affecting the irreducible water saturation were fractal dimension, maximum connected pore throat radius, minimum pore throat radius, and thickness of the water film. The calculation results are closer to the experimental value when the fractal dimension is greater than 2.7. This comprehensive application of various experimental and theoretical methods gives a better understanding of the pore structure characteristics and fluid distribution in tight reservoir samples.