Abstract: Long-term water flooding will inevitably lead to the change of the pore throat characteristics. And the calculation of the pore throat size is the key to blocking water flow preferential passage by profile control and flooding system. Based on the CarmanKozeny formula, together with the numerical simulation, a method of calculating the reservoir pore throat size was established. The numerical simulation was used to match the oilfield production, calculate the present interwell permeability. The results calculated show that the change of pore size in the B oilfield is basically the same as the estimated by experience and tracer test. Compared with other traditional methods, it is convenient and inexpensive. The calculation results can provide guidance for the subsequent profile control design and system size selection, promotes the field application.