Abstract: The position of the bottom water breakthrough in horizontal well production was studied by means of the reservoir numerical simulation software. Based on the production formulas for horizontal wells, the production pressure difference corresponding to the vertical plane radial seepage resistance was deduced. Assuming the vertical plane production pressure drop to satisfy the logarithmic distribution, the bottom water coning rate with consideration of effect of gravity can be obtained, by which the equation of water breakthrough time of oil well is derived.This method’s reliability was verified by the case study. The research shows that bottom water breakthrough starts from the middle of horizontal well section, and the longer the production time, the faster the rate of bottom water coning. Reasonably increasing the length of horizontal well section and the height of water avoidance can postpone the time of water breakthrough in oil wells