Among all the available natural resources for mankind, water is the most essential resource for the sustainable development of human society and for basic support of all human and natural systems. However, the problem of how to measure water sustainability properly or how large scale one certain amount of water resources can support the social, economic, ecological, and environmental system in one region by means of quantifiable methods is the most serious issue among many matters including sustainable management of water resources. A scenario analysis to analyze the water sustainability of Chongming Island, the largest alluvial island in the world, was performed. A serious situation is found that the growth rate for water demand is much higher than that of water supply. A policy of rapid socioeconomic development and better environmental protection is recommended to achieve the most sustainable development of Chongming Island in the future. The research work suggested that water sustainability analysis is a powerful and sample approach for the assessment of sustainable regional development.