Abstract: Achieving a healthy regional water balance is an important content and challenge of China′s water conservancy work, which is of great significance to ensure national water security and the sustainable and stable development of the national economy and society. This study defined the concept of a regional water balance based on a summarization of the fundamental understanding of a water balance, then its connotation was interpreted from four aspects: ① water budget balance; ② water supply-demand balance of economic society; ③ water use balance between the economy and ecology; ④ harmonious balance of human-water relationship, and three dimensions of time, space, system. This study proposed the basic principles of a regional water balance, including water balance principle, supply and demand balance principle, synergistic balance principle and harmonious balance principle. The important scientific challenges for realizing a regional water balance supported by basic principles were analyzed. The theoretical system of regional water balance was discussed in terms of two aspects of theoretical basis and basic theory. The theoretical basis includes hydrology, science of water resources, ecology, economics, sociology and other theories. The basic theory includes the spatiotemporal equilibrium theory of water resources budget, the optimal allocation theory of water resources supply and demand, the synergy theory of water use between the economy and ecology, and the human-water harmony theory. This study also discussed the prospect of applying the basic theory. The results of this study can act as a reference for future studies of the regional water balance and can provide a research framework for management of the regional water balance in the future.