Urban water environment problems involve three major processes: development control at the source, management and supervision in the process, and water treatment at the end-of-pipe. However, the current water environment protection and management fails to clarify the causes of water environment problems as well as differentiate the boundaries of responsibility behind these phenomena. In this regard, based on the perspective of "land-water coupling", this study introduced the three concepts of "effect value", "measured value", and "target value" of water quality, and constructed a system of urban development capacity assessment and water quality problems diagnosis according to the analysis of "tri-value linkage", and Hangzhou was taken as the example to conduct empirical analysis. The results showed that: (1) According to the three principles of watershed singularity, unity of water quality control objectives, and uniqueness of administrative subjects, the water management units, which were divided by sub-basins, water quality control sections, and administrative boundary data, effectively clarified the responsibility boundaries between the various local management departments. (2) 82.6% of the research units showed a surplus in water environment capacity (effect value < target value), which indicated that their source development control was effective; The rest of the research units, whose water environmental capacity were insufficient or even seriously exceeded (effect value > target value), had some problems in their source development control, which were mainly located in the area south of Qiantang River. 67.4% of the research units had reached the water quality standard (target value>measured value), which directly indicated that their end-of-pipe water treatment was effective, and these units were mainly located in the west and north of Hangzhou; The units whose water quality exceeded the standard (target value < measured value) still need to be perfected in their end-of-pipe water treatment, which were concentrated in areas like Xiaoshan Science and Technology City, Qianjiang Century City, Xianghu New City and other areas. 87.2% of the research units had problems in process management supervision or end-of-pipe water treatment (effect value < measured value), 68.0% of which had effective end-of-pipe treatment but their process management supervision still had some problems (target value>measured value), and the remaining 32.0% of them had defects in both process management supervision and end-of-pipe water treatment (target value < measured value). (3) The assessment and diagnostic system is able to monitor the dynamics of land development capacity and water quality constraints in real time, and provides a new decision-making tool for land-water coupled spatial use control of the national territory.