Mid-high permeability sandstone reservoir is one of the main oil and gas areas of the high water-cut mature oil reservoirs in China, and its physical properties and seepage capacity are controlled by the micro-pore structure. The quantitative classification and evaluation of micro-pore structure is significant tothe accurate development of oil fields. Based on micro-nano CT scanning experiments of class I, class II and class III reservoirs in Songliao Basin, a 3D characterization method of microscopic pore structure was developed, and a classification method of pore structure based on comprehensive evaluation of storage space and flow capacity was established to realize the classification and evaluation of pore structure characteristics of different types of oil reservoirs. The results are as follows. The pore structure and size distribution of class I, class II and class III reservoirs are significantly different, the pore diameters of which are about 50 μm, 32 μm and 18 μm respectively. A micro-pore structure classification method is proposed by taking flow flux area and pore diameter as evaluation parameters, which divides pore structure into 6 types: high-connected large pore, low-connected large pore, high-connected medium pore, low-connected medium pore, high-connected small pore and low-connected small pore. Based on the 3D core reconstruction model, the dominant pore structure type of class I, class II and class III reservoirs are evaluated to be high-connected medium-large pore, high-connected small-medium pore and high-connected small pore respectively, which reveals the differences of micro-throat connectivity. The research provides the basis for the study of the microscopic remaining oil occurrence state and the optimization of EOR methods in the late period of ultra-high water cut.