In recent years, a set of strata with loose structure and worse self stabilization, which named unconsolidated sandstone, has been exposed by several large underground engineerings with frequent engineering accidents, such as gushing water, collapse and other problems. Analyzing its origin and occurrence environment is of great significance both in understanding the essence of its special properties and dealing with the relevant engineering issues. Combined with sedimentary environment of the Neogene in Gansu, the origin of the unconsolidated sandstone, which can be comparing with modern desert aeolian sand and beach sand, is studied by using grain size analysis, scanning electronic microscope (SEM) and X ray diffraction (XRD) by which its distribution characteristics, occurrence and exposure characteristics, grain size characteristics, quartz sand grain surface characteristics and cementation characteristics, would be discussed in this paper. The analysis result shows that the unconsolidated sandstone, which belongs to the parts of the Neogene red layers in Gansu province, has the same occurrence and exposure characteristics as adjacent strata. And this sandstone has well sorted, large porosity and less clay minerals. Its quartz grain surface shows some butterfly shaped craters, surface pockmarks and siliceous sediments. All of those characteristics can be inferred that the unconsolidated sandstone in Gansu is the wind-sand activity product with dry and hot climate in late Neogene, and parts of the strata are influenced by water environment afterwards.