Reservoir heterogeneity greatly affects subsurface fluid flow and distribution and thus should be the leading factor in study of hydrocarbon migration and enhanced oil recovery. The differential diagenesis in clastic rock reservoir is an important element for innerreservoir heterogeneity, and its interaction with hydrocarbon charge is also the core for study of the hydrocarbon accumulation mechanism of tight sand reservoirs. This study took the 9th Member of upper Triassic Yanchang Formation in the central and western parts of Ordos Basin as an example to understanding the differential diagenesis of sandstone reservoirs and the process of subsurface fluid. Micrographic observation illustrates that the reservoir of the 9th Member of Yanchang Formation consists of four types of sandstone, namely ductiledebrisrich tight sandstone, calcitecement tight sandstone, water bearing sandstone and oil bearing sandstone. Integrated analyses, using micrographic and ultraviolet fluorescence spectra analyses, homogenization temperature test of fluid inclusions and basin modeling, show that consolidation at the early stage of diagenesis resulted in densification and plastic deformation of the mechanically compacted sandstone, while strong carbonatic cementation at the early stage of diagenesis resulted in compaction of calcitic cemented dense sandstone. The sandstone of these two kinds can not provide pore space for fluid activity at the late stage of diagenesis. The waterbearing sandstone and oilbearing sandstone had experienced differential and multiphase diagenesis. At least three bitumens have been identified within oilbearing sandstone while waterbearing sandstone found no bitumen residue. Using bituminic matter in oilbearing sandstone as a mark of charging of hydrocarbon, diagenetic sequence of the 9th Member of Yangchang Formation was reconstructed and three fluid flowing processes have been classified, with hydrocarbon charging and diagenesis occurring alternatively. Waterbearing sandstone experienced roughly similar diagenesis process, but no hydrocarbon charging occurred in the waterbearing sandstone.