Abstract: Using experimental data of constant-rate mercury injection, the paper studies the microscopic pore structures and oil and water distributions in low and extra-low permeability reservoirs in Yanchang oilfield. The results show that throat dominates the development effect. Average radius of throats and number of effective throat per unit volume control the reservoir permeability collectively. In the sequence of effective throat volume per unit volume, main current throat radius, the max. connecting throat radius, their influences on the permeability decrease, which has nothing to do with the mercury injection saturation of throats. The pore throat radius ratio correlates poorly with porosity, but it has a strong negative correlation with permeability. With the decrease of pore throat radius ratio, the development degree of throat becomes better and the fluid permeability increases. Therefore, the oil and gas in the formation are easily displaced through throats, but the correlation between the ratio of pore throat radius and porosity is not obvious and the ratio of pore throat radius does not vary with the change of porosity regularly