Chang 6 reservoir ofUpper Triassic is the major oil producing zone in Baibao-Pingzhuang area, Ordos Basin. It is also typical low porosity and low permeability reservoir, with fine grain size, low compositional maturity, poor psephicity and good sorting. Based one these lithologic characteristics, the tests combining with the analysis of cast slice, SEM, X-clay minerals diffraction and property are applied to analyze the diagenesis of Chang 6 sandstone reservoirs. It shows that the diageneses mainly include compaction, pressure solution, cementation, packing action and dissolution. The first four actions are destructive diageneses, and the last one is constructive diagenesis. According to the evolutionary sequences of the diagenesis pattern, it is concluded that the diagenetic evolution of Chang 6 sandstone has already been in earlier stage of the middle diagenetic stage B. Based on the combination mode of the diagenetic evolution pattern, the evolutionary diagenetic pattern is studied. By emphasizing the process of the diagenetic evolution, the diagenetic facies is researched detailedly, and it is divided into seven types.