Abstract: The study of reservoir microscopic pore structure is an important part in comprehensive reservoir evaluation.Based on slice observation,casting thin section and conventional constant-pressure mercury injection,combined with constant-speed mercury injection,the comprehensive analysis of ultra-low permeability sandstone reservoir from Chang8_2 reservoir in Zhanghan area of Ordos Basin was carried out.The results indicated Chang 8_2 reservoir was of multiple storage space,the main type was of primary intergranular pore and the pore structure could be divided into three categories:low displacement pressure-medium throat,medium displacement pressure-fine throat,high displacement pressure-micro-fine throat.The reservoir pore structure was complex and various,the expulsion pressure and median pressure were on the high side,while the pore connectivity was poor.According to reservoir pore structure characteristics,the reservoirs were divided into typeⅠ,typeⅡ and typeⅢ.Comprehensive analysis indicated due to small flow pore radius,poor flow capacity and high reservoir starting pressure,high productivity could be only obtained through fracturing.In development process of low permeability reservoir of Chang 8_2 reservoir,the protection of throat reservoir should be paid attention to so as to achieve better development effect.