To study the relationship between the economic system and the water environmental system in Beijing, China, this article brought in the concept of "system coupling", and calculated their coupling degree using the co-coordination model. Specifically, the economic system consists of population and economic development subsystems; the water environmental system consists of water supply and water environmental carrying capacity subsystems. A total of twelve components were chosen, including population, water consumption per ten thousand Yuan GDP, etc. Besides, the Structural Equation Model (SEM) was innovatively used to analyze the micro-coupling paths among the subsystems. Finally, we drew conclusions from the coupling state perspective and coupling paths perspective, respectively. From the macroscopic point of view, there are complex interactions between the economic system and water environmental system in Beijing, and their coupling degree will increase with the optimization of economic structure and advancement in technologies. From the microscopic point of view, there are also complex interactions among the four subsystems in Beijing. Especially, the water supply subsystem has a certain degree of promotion effect on the other three subsystems.