The lower pore and throat radius parameters from conventional mercury penetration technique affect the reservoir evaluation and later development technique policy constituting. Based on constant-rate mercury penetration technique, the microcosmic pore throat characteristics of tight reservoir of Yanchang Formation of Ordos Basin were analyzed, and the alterations of pore throat characteristic parameters were also quantitatively and clearly characterized. The average pore radius of tight reservoir is about 153 μm and has no correlation with the permeability. The average throat radius is about 0.34 μm and there is a great positive correlation between the throat radius and permeability. The average ratio of pore and throat radius is 556, and there is a negative correlation between the ratio and permeability. While mercury enters into the bigger pores controlled by wide throats for little resistance, the influence of throats is unclear. With input mercury gradually increasing and capillary pressure rising, throats gradually control whole mercury saturation. Using advanced technique to change reservoir formation to increase or enlarge throat radius and decrease the ratioofpore and throat radius, in order to increase permeability, so the development of tight oil can have a good result.