Abstract: Based on casting thin sections,SEM and X-ray diffraction tests,the research on petrology characteristics and diagenesis of Chang 6_3 reservoir in Hua Qing area of Ordos Basin was carried out.The results show the reservoir in the study area was mainly composed of arkoses and lithic arkoses and it had little feldspar lithic sandstone.The main diagenesis includ compaction,cementation and dissolution.The compaction is the main reason of poor reservoir physical property,which can reduce the reservoir porosity by 23% on average.The cementation can make the reservoir physical properties become further worse while the dissolution can improve reservoir properties and increase the reservoir surface porosity by 0.7% on average.Besides that,the results indicate the diagenesis of Chang 6_3 reservoir in Huaqing area mainly belongs to the late of A middle diagenesis and partly belongs to the early of B middle diagenesis.