Abstract: Tight oil is an important component of unconventional oil and gas resources. The pore structure of tight oil reservoirs determines flow. In this paper, two kinds of mercury injection, pressure-controlled and rate-controlled, were used to carry out a quantitative characterization on eight tight core samples. The characteristics of pore structure characterization of the two methods were clarified one by one. The capillary pressure curves of eight samples were calculated on the same chart combining the two methods, and the pore size distribution histograms of eight samples were calculated according to Loucks classification method. The results show that: (1) At low mercury saturation, the total curve of rate-controlled mercury intrusion and the mercury injection curve of pressure-controlled mercury intrusion overlap well, indicating the same pore structure. (2) Pore size distribution curves show that the pore radius of tight oil reservoirs ranges 9.2 nm-500 μm with a multimodal pattern. There is a peak at 80-500 μm diameter, and the peak of the radius is about 150 μm. Nano pores with the pore radius of less than 1 μm developed well with multiple peaks. (3) According to Loucks' classification method, nano pores (<1 μm) are the main pore type, meso pores larger than 62.5 μm the second, and micron pores the least.