The contradiction between pressure recovery and water breakthrough in the KT-II reservoir of B oilfield has been prominent since water injection development in 2013. In order to accurately evaluate the effectiveness of oilfield water injection development, it is necessary to use the water consumption index. The water consumption index provided by existing literature is derived from the A-type water drive curve, and a sandstone reservoir is selected for example analysis. Due to the fact that the KT-II reservoir is a weakly volatile carbonate reservoir, the initial water consumption index calculated using existing methods is higher than the actual water consumption index of the reservoir. This article addresses the shortcomings of existing methods, starting from the basic concepts and mathematical expressions of water consumption rate, water consumption index, and water storage rate. Firstly, establish the expressions for water consumption index and water storage rate. Secondly, choose the equations for IPR and six water drive curves, and substitute into the expressions for water consumption index and water storage rate. Finally, a water consumption index calculation chart based on water content is established. The key influencing parameter of water consumption index obtained through chart is water content, among which the IPR chart is most suitable for the current water content stage of KT-II reservoir; This method can more accurately predict the water consumption index of weakly volatile carbonate reservoirs compared to existing methods. This method is simple, reliable, easy to master, and has instructional significance for evaluating the water consumption index of the same type of reservoir.