A novel early-warning method of water resource carrying status (WRCS) is proposed in the current work and applied in Nanjing City, China. Specifically, the concepts of ultimate water resource carrying capacity (UWRCC) and WRCS are first introduced to differentiate between various methods of determining water resource carrying capacity (WRCC). A coordinated development index (CDI) between the water resource carrying subject and carrying object is then introduced as a warning indicator of WRCS. The warning levels are determined by combining CDI levels and changes in CDI values. The novel method offers more warning information than conventional early-warning methods. The early-warning results can help decision-makers better understand the deficiencies of water resources in supporting socioeconomic development. In the analyses presented, the CDI is calculated for each district in Nanjing for the 2015, 2020, 2025, and 2030 years, and the warning levels for WRCS under future scenarios (the years 2020, 2025, and 2030) are determined. All districts had a warning level I in 2020. However, by 2025, the warning levels for Urban district, Jiangning, and Pukou, were projected to change from I to II. Furthermore, by 2030, Jiangning would have the most severe early-warning results, with a warning level of IV. Such adverse results are mainly due to the large socioeconomic scale and high water demands, which should be considered in Nanjing’s future development plans.