Abstract: Reservoir sand stone of chang 6 and chang 8 in Yanchang Formation of Zhengning area in Ordos basin is typical of low porosity and low permeability,which has the characteristics of finegra in size,low compositional maturity,poor roundness and sorting.Based on casting sliceidentification of sand stone, scanelectron micro scope,X-ray diffraction,reflection of vitrinite and physical property,we studiedd iagenetic characteristics of Yanchang Formation chang 6 and chang 8,analyszed their effect on reservoir physical property,and determined diagenetic stage of reservoir sand stone diagenetic stage of reservoir sand stone is mainly the lated iagenetic phase A.Our study shows that mechanical compaction and late cement of carbonate are the main factor causing the loss of porosity and reducing the reservoir quality of the sand stones.