Water resources serve as the material base of the existence and development of human beings.It is a huge complicated system involving population,economy,society and environment.Water resources and water cycle are the keys in the system.Based on the summarization of the water resources concept and its attribute of nature,economy and society,this paper discusses the relationships between water resources and water cycle,water-balance structure,population,economy,society and environment in details.The paper also emphasizes the interactions of water cycle and water balance,water resources utilization and evaluation,economic activity,and water resources carrying capacity.Given an example of the Yellow River valley,the paper states that there is a closer relationship between water resources,population,economy,society and environment.The author gives a definite concept and connotation of water resources carrying capacity on the base of summing up the former studies.Summarizing the model system of water resources carrying capacity,the author also points out that it is very essential to use the system to depict the complex relationship between water resources and other four aspects,namely population,economy,society and environment.Considering the problem from the viewpoint of the regional sustainable development and alleviating the benefit conflict of distribution of water resources,we should clarify the following aspects:water demands for ecological environment,including rivers,animals,plants and land;water demands for population and social-economic development;and water environment capacity that can tolerate certain amount of pollutants.So,in order to promote the sustainable development of population,economy,society and environment,it is of great significance to simulate and predict the evolvement of the huge complex system with the combination of remote sensing,GIS and modern math theory,seek a reasonable allocation of water resources carrying capacity,and study the enhanced module of water resources carrying capacity.