Abstract: Based on the theory of seepage mechanics, a model for calculating water breakthrough time of the bottom water reservoir is established, and the formulas of water breakthrough time of oil wells in the bottom water reservoir without interlayer and with interlayer are deduced. Combined with the actual geological characteristics of bottom water reservoir, the values of three parameters in the formula are demonstrated, including porosity, oil saturation of two-phase flow interval and permeability coefficient of reservoir. The water breakthrough time of three wells with interlayers is calculated by the derived water breakthrough time formula. Compared with the actual water breakthrough time, the relative error is no more than 9% and calculation accuracy meets the actual requirement of oilfield development.