Abstract: The reservoir of upper Triassic Xujiahe formation in northwest Sichuan is mainly feldspar quartz sandstone and feldspa lithic quartzite sandstone. The reservoir sandstone is typical low porous and low permeable reservoir rock. Based on core observation, this article took research and materials analysis on thin section observation, scanning transmission electron microscope observation, cathodolum inesconce and physical property, which indicated that the sandstone of upper Triassic Xujiahe formation in northwest Sichuan went through the diagnesis stage of compaction, cementation, metasomasis, solution and fracturing, and now it is in the late diagenesis stage B period. The di-agenesis has the greatest effects in controlling physical property in this aera. During the porosity evolution, the compaction and the cemetation mainly played the role of destruction, the solution and the fracturing played the role of construction.