The microscopic pore structure of Chang 3 ultra-low permeability reservoir of the Yanchang Formation in the Ordos Basin Fuxian exploration area is thoroughly analyzed and investigated by using constant-speed mercury intrusion technique, combined with conventional casting thin sections, scanning electron microscopy, constant-pressure mercury injection and other analytical approaches for the first time. The main pore type of the reservoir is residual intergranular pore and dissolved intergranular pore. The pore structure is divided into five categories: lower displacement pressure-coarser throat type, low displacement pressure-coarse throat type, middle displacement pressure-middle throat type, high displacement pressure-fine throat type, higher displacement pressure-finer throat type. The result of the constant-speed mercury intrusion test shows that the distribution of the pore radius ranges from 100 to 200 μm,the pore is mainly the medium-small, few samples exhibit the large scale, and pore distribution curves are partial coarse, the distribution of throat radius ranges from 0.2 to 0.7 μm, the throat is mainly micro-throat, throat distribution curves are partial thin, reservoir flow capacity has poor correlation with the pore size, and it is mainly controlled by the size of the average radius of throat, distribution patterns of throat-size, and thus affecting the development effectiveness of super-low permeability and ultra-low permeability reservoirs.