The Upper Paleozoic gas reservoir in Ordos Basin is tight sandstone gas reservoir. Through the analysis of the source rock, the reservoir, the timing of gas accumulation and reservoir tightening and the gas migration, the accumulation conditions of the Upper Paleozoic tight sandstone gas in the basin is studied. The coal bed in the Benxi Formation-Shanxi Formation is the main source rock. The kerogen is the type Ⅲ kerogen, the organic matter abundance and maturity are high, which supplies enough material. The reservoir lithology is mainly lithic quartz sandstone, the quartz sandstones and lithic sandstone. The porosity is mainly 4-8%, the permeability is mainly 0.1-0.5*10-3μm2. The reservoir is low porosity and low permeability, with small pore, fine throat, little microfracture, poor connectivity of the pore and throat. The gas reservoir is accumulating after the tighting, the reservoir becomes the tight sandstone reservoir in early-middle Jurassic, the gas reservoir accumulates in late Jurassic-early Cretaceous. Under the force of gas expansion, the gas migrates in short distant along the microfracture, then forms the widespread tight sandstone gas reservoir.