Abstract: After studying the diagenesis sequence about the sandstone gas bed of the Sulige gas field in Western Ordos Basin, the petrography of organic fluid inclusions has been studied through carrying out petrographical observation and fluid inclusion microthermometry under microscope, the homogenization temperatures and salinities of these organic fluid inclusion shave been analyzed, and the formation characteristics of gas reservoir in the Sulige gas field have been discussed in this paper. Results indicate that the sandstone of gas bed had experienced diagenesis of compaction, pressolution, cementation, quartz secondary increase, dissolution and fracturing.According to the distribution characteristics and the physical phase state of organic fluid inclusions, two stages organic fluid inclusions have been identified by microscopeob servation. The first stage organic fluid inclusions mainly are oil/gas inclusions, which are featured with main phases, including gaseous hydrocarbon and brine, gaseous hydrocarbon and liquid hydrocarbon together with brine or gaseous hydrocarbon and liquid hydrocarbon.For the first stage organic fluid inclusions, their volumes of bubble are relatively small, and their homogenization temperatures are relatively low with the main peak around 120℃.These features of first stage organic fluid inclusions represent the formation and initial migration into reservoir of natural gas at early stage.The second stage organic fluid inclusions are characterized with mainly pure gas phase, with a small amount of brine. For the second stage organic fluid inclusions, their volumes of bubble are relatively big, and their homogenization temperatures are relatively high with the main peak around 150℃.The second organic fluid inclusionsare mainly distributed in the dissolution pores or filled in cracks.The second stage organic fluid inclusions stand for the gas formationpeak and large scale migration and accumulation, representing the most important reservoir formation event in this studying area. The T-t curve of chronology shows that the gas initially migrated into reservoir of natural gas since about 170Ma(Middle-Late Jurassic) and the gas accumulated peak happened at about 120Ma (late period of Early Cretaceous).