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To understand the production decline law and accurately calculate the decline rates at ultra-high water cut stages, the calculation formula is deduced using the theory of oil-water two-phase flow and the reservoir engineering method based on the relative permeability curve that reflects the nature of oil-water two-phase seepage flow. Through comparison with the actual production data in the ultra-high water cut period, the accuracy of the formula is verified and the analysis on the influencing factors of the production decline rate is conducted. The results show that with the increase of parameters such as absolute permeability, pressure difference between injection well and production well and the change rate of oil relative permeability, the production decline rate increases almost linearly. With the increase of row distance, the production decline rate decreases. The effects of absolute permeability, pressure difference between injection well and production well, and the change rate of oil relative permeability on the production decline rate are the same, while the row distance effect is the most significant. The research results indicate that the proposed formula can precisely calculate the production decline rate at the ultra-high water cut stage, and this production decline rate follows an exponential law. [ABSTRACT FROM AUTHOR]