When a production well adjusted to an injection well, the water injectivity index of the well will keep increasing with the oil near the well bottom being droved away by the injected water. However, there is no accurate formula to describe the law so far. Based on the Buckley-Leverett equation and the classical Darcy equation, combined with the relationship curve of water cut and water saturation, the relationship formula of the injectivity index per meter and cumulative water injection per meter was deduced. Finally, the field data was used to analyze and verify the validity of the formula. This formula provides a theoretical basis for the future calculation of injectivity index per meter and the well injection allocation.