Abstract: According to the analysis and testing data from sandstone thin?section, cast thin?sections, cathodoluminescence, scanning electron microscope, X?ray diffraction, the diagenesis of Chang?9 tight sandstone reservoir in Ansai area in Ordos basin is studied. The results show that Chang?9 sandstone reservoir went mainly through compaction, cementation and dissolution, of which the compaction and cementation are the main factors for reducing reservoir porosity, the dissolution is the main factor for increasing reservoir porosity, while the microfractures serve as improving the reservoir permeability and increasing the fluid percolation ability in a certain extent. The Chang?9 sandstone reservoir’s diagenesis belongs to the“A”phase of medium diagenesis, with close relations between porosity evolution and diagenesis. The compaction made the porosity loss of 15.64%, the cementation made the porosity loss of 11.32%, the dissolution and micro?fractures increased porosity of 4.04%, hence the calculated porosity in the study area is determined as 7.48%, which is relatively consistentwith the average value of the measured porosity of 7.20%. The favorable diagenetic facies of Chang?9 reservoir is the feldspar corrosion facies and the chlorite film?feldspar corrosion facies developed in the underwater distributary channel sedimentary microfacies, and the most unfavorable diagenetic facies are the carbonate cementation facies and the strong compaction facies