Abstract: The reservoir of Chang 8 formation in southeast Ordos Basin is a set of lake-delta sediments and the dominant reservoirs. The main rock type of the reservoir is arkose and feldspathic lithic sandstone. Based on core observation, combining with thin section observation, scanning transmission electron microscope , pressure mercury testing, X ray diffraction analyses, measurement of fluid inclusion, reflection of vitrinite measurement, it is believed that the diagenesis includes compaction, cementation, dissolution and alteration. And the diagenesis reaches A-period of middle diagenetic stage. The diagenesis is a key factor for controlling the physical property in this area. Most primary pores were destructed after the compaction and cementation in early diagenetic stage. The property of reservoir is improved by the dissolution of instable mineral like feldspar. The dissolution is due to the organic matter-decomposed acid fluid. But the reservoir become dense because of the ferroan calcite cementation in late diagenetic stage. The feature of the pores evolution in the diagenesis has positive influence on the development of low permeable reservoir in the southeast Ordos Basin.