Based on the observation of core, casting thin section and scanning electron microscope and analysis of physical property, static characterization and dynamic evaluation of micro pore structure of carbonate reservoir of Majiagou Member 5 in northwestern Ordos Basin are carried out by means of high pressure mercury injection test, gas water relative permeability test and nuclear magnetic resonance test. The results show that the lithology of Majiagou Member 5 in northwest of Ordos Basin is mainly dolomite with low porosity and low permeability. The reservoir spaces with great significance for oil and gas reservoir and seepage are inter-granular solution pore, solution pore, mold pore and fractures. According to mercury injection parameters and capillary pressure curve shape, the pore structure types of Majiagou Member 5 are divided into four types. Eight parameters, such as porosity, permeability, main pore throat radius, maximum mercury saturation, relative permeability at the same permeability point, movable fluid saturation, gas testing production and types of pore throat assemblage, are selected to establish the classification and evaluation criteria for the micro pore structure of carbonate reservoir of Majiagou Member 5. The solution pores, inter-granular solution pores and fractures of class Ⅰ and class Ⅱ reservoirs are well developed, with medium and large pore throat, porosity＞5.5，, permeability＞0.1×10-3μm2, maximum mercury saturation＞70，, relative permeability at the same permeability point ≥0.05×10-3μm2, movable fluid saturation＞60，. Reservoirs of class Ⅰ and Ⅱ are the main goals of exploration and development, in which industrial gas flow can be obtained through fracturing transformation.