Abstract: Hydrocarbon charging threshold is the basis for tight oil accumulation research. The microstructure of Chang 8 tight sandstone reservoirs in the Fuxian area of Ordos Basin is very complicated, which is a key factor influencing hydrocarbon charging. Field emission scanning electron microscopy (SEM) and CT methods were used to quantitatively study the pore-throat characteristics of tight sandstone reservoirs. Mechanical properties, well logging and effective pore-throat analyses were used to determine the hydrocarbon charging threshold of physical properties and pore-throat characteristics. The results showed that the pore size of tight reservoirs is dominated by nanometer and submicron scale, and the average pore radius is 1.2 μm. The average radius of throat is 0.1 μm, which belongs to micropore-fine throat reservoirs. It was calculated that the charging threshold of porosity of Chang 8 tight sandstone reservoirs is 4.5%, and that of permeability is 0.04×10^-3 μm². The threshold of pore-throat diameter on the source and reservoir interface is 15.77 nm, and that inside reservoir is 24 nm. We integrated that oil and gas might occur in nano-scale pores as oil bead, throat or thin film as compaction and cementation effects became stronger.