Based on the normal thin sections, scan electron microscope and X-ray diffractions, a study was carried out on the characteristics of petrology and diagenesis of Chang 6 reservoirs in Yangqi area, Ordos Basin. Research shows that fine arkose sandstone are the main sandstones of the Chang 6 reservoir. The main granular pore and secondary feldspar-dissolution porewhichare mainly composed of small hole and micropore. The Chang 6 reservoirs has super low permeability and is accessed densify reservoir of which the permeability is less than 5×10-3 μm2. The strong compaction and cementation are the main factors which cause the low porosity and low permeability of the reservoir, while dissolution is the main factor for improving the physical property of the reservoir.