Abstract: The He 8th member in the Ordos Basin is a typical field explored for tight sandstone gas. Studies have been carried out to figure out the microscopic characteristics and forming mechanisms of the tight sandstone gas reservoir. After observation of cores, a variety of experiments were carried out, such as cast thin sections, field emission scanning electron microscope, CT scanning and constant-rate mercury injection. It is concluded that the tight sandstones in the He 8th member are quartz sandstone and lithic quartz sandstone, which are moderate to coarse, moderately sorted, angular and subangular, point or line contact among grains and filled with various cements. The reservoir space is composed of residual intergranular pores, dissolution pores, micro- and nano-pores and fractures, and micro- and nan-pores contribute over 90% to the reservoir porosity. Throats are contracted necks, curved sheets and tubes. They are thin and commonly connected. The average throat radius is well correlated to permeability. Sedimentation and sedimentary structure control the difference in original reservoir physical properties. The central sandstone in the braided river and the delta distributary channel have better physical properties than those of the channel sandstone in the alluvial fan. Late strong compaction tightened the reservoir, cementation damaged the pore structures, and strong dissolution improved the reservoir properties to some extent.