On the basis of petrologic study of about 40 wells, this paper expounds the characteristics of the Paleogene and Neogene sandstone reservoir in southwestern Qaidam Basin： (1)The rock composition of sandstone reservoir varies regularly in space-time. The compositional maturity and textural maturity of sandstones located in both sides of southeastern and northwestern in the study area are obviously lower than that located in the middle area. The compositional maturity and textural maturity of sandstones decline gradually from Paleogene to Neogene. This is accordance with the fact that the regional tectonic activity in the study area was enhancing from early to later. The variation of sandstone reservoir petrology affects its reservoir quality clearly, and then it makes the sandstone reservoir quality in both sides of southeastern and northwestern in the study area getting worse. (2) Primary pore is mainly types of pore in sandstone reservoir. Fore-researchers have different views on the pore types of sandstone reservoir, it is mostly believed that the main types of pore is secondary pore, especially in lower Ganchaigou formation, mainly developed secondary pore. The sandstone reservoir of Paleogene and Neogene is mainly for primary pore, and next for cement dissolution pore and rock debris dissolution pore, and secondary pore only improve the sandstone’s reservoir quality limitedly. So research for development regularity of primary pore in sandstone reservoir is more important. (3) The chief factors resulting in reservoir quality change are mainly due to the difference of compaction, and then to the cementation .The compaction degree is affected by buried depth and rock property, and also related to the buried way undergoing by the reservoir. The cementation in sandstone reservoir is mainly carbonate and anhydrite and fewer analcite. The content of carbonate cements varies between 4.0% and  16.0% in different layers., locally the content of anhydrite cement may be up to 11.5% to 28.0%, and it becomes primary factors affecting the reservoir quality.