Abstract：Based on researches by means of ordinary thin sections, cast thin sections, scanning electron microscope and X-ray diffraction, the authors studied the petrologic and diagenetic characteristics of the Paleogene-Neogene sandstone reservoirs in southwestern Qaidam Basin, and the result shows that sandstone reservoirs in this area are mainly composed of feldspar lithic sandstone, lithic sandstone and lithic arkose and have mainly experienced such actions as compaction, cementation, corrosion and metasomatism, with the evolution reaching the middle diagenetic stage B. Compaction and cementation at the early diagenetic stage seem to have been the main diagenetic types which caused the destruction of primary pores, whereas corrosion at the middle diagenetic stage was probably the main diagenetic type resulting in the induced pores. Controlled by the diagenesis and the diagenetic phase, the induced pore development belt usually appears in relatively deep reservoirs, and the high abnormal belts of porosity and permeability are developed in deep reservoirs.