Abstract: Water allocation of trans-province rivers is the basis and premise to manage the conflicts between human water demands and natural water demands and the clashes among water uses by different provinces. It is essentially a type of benefit allocation that should be based on both the natural sciences and social sciences. This paper discusses several basic principles for water resources allocation based on the theory of justice in politics and the theory of property rights in economics, and develops a quantitative model for water allocation of trans-province rivers, considering water supply-demand relationship and hydrological characteristics. Applications of this model to case studies of the basins of Wuding River and Wei River demonstrate its uses in two scenarios of water surplus and water shortage.The results show that it meets the basic human and natural water demands, and its optimized allocation scheme can alleviate the social conflicts and is easy to implement. Thus, it helps better balance the conflicts of domestic water use, economic water use, and ecological water use among the riparian provinces.