Carbonate fracture-vuggy reservoirs exhibit strong heterogeneity. For the cavity reservoirs, water injection is commonly used for further development after depletion-drive recovery. Currently, a water injection index curve equation suitable for calculating dynamic reserves of sealed cavity reservoirs has been established, which can be used to calculate dynamic reserves by the slope of the curve. However, the traditional water injection index curves only consider the elastic compression factor of crude oil, which is applicable in the early stage of water injection development, and the calculated values gradually become larger in the middle and later stages. Based on the original water injection index curve, this paper comprehensively considers multiple factors such as the elastic expansion energy of crude oil, rocks, bound water, bottom water, and the secondary bottom water formed by multiple rounds of water injection to derive an improved water injection index curve equation suitable for dynamic reserve calculation of any water injection cycle in the sealed cavity reservoirs. By incorporating cumulative oil and water production at the wellhead, the equation is simplified into two major categories (sealed constant-volume without water and sealed constant-volume with weak aquifer) and six simplified equations for different development stages. Calculation results of typical single well dynamic reserve calculation show that the improved water injection index curve is suitable for any stage of water injection development with broader applicability. This method has been widely applied in measures tapping and block comprehensive adjustment of Harahatang Oilfield in Tarim Basin, demonstrating good implementation results.