Abstract: The study on microscopic pore structure is not easy and it plays an important role in low permeability reservoir research,which directly affects the accuracy of reservoir evaluation.By taking low permeability reservoirs of Chang-3member from Yanchang formation in Xunyi- Yijun area,Ordos Basin as the example,based on the study of thin sections,rock porosity and permeability,capillary pressure test and well log analysis,the study on reservoir microscopic pore structure types and characteristics has been carried out and therefore,the reservoir is divided into four kinds of pore structure.The aspects of reservoir space types,porosity and permeability characteristics,capillary pressure curve characteristic differences have been fully clarified.Type-I and type-II reservoirs are considered to be favorable,the reservoir quality index and reservoir microscopic pore structure characteristics parameters(displacement pressure,average pore throat radius and sorting coefficient)have been chosen to make reservoir evaluation. The evaluation results of WB44 and other 4 wells indicate the pore structure type has a direct effect on capacity evaluation and log interpretation of low permeability reservoir.