Abstract: Based on such means as thin section identification, scanning electron microscope and X-ray diffraction, the authors studied reservoir lithology, types of diagenesis and influence on pore evolution of Chang 6 reservoir in southeast Ordos Basin. The main sandstone types of Chang 6 reservoir are arkose and lithic feldspathic sandstones. Most sand grains are round-subangular and moderately sorted. The sandstone belongs to stage A of middle diagenetic stage. Diagenesis includes compaction, cementation, dissolution and metasomatism. Compaction and cementation reduced initial pore volume by 68.8% and 16.90% respectively. Later, dissolution increased pore volume by 3.67%, which was useful to oil and gas accumulation.