Abstract: Three lines and one list is an important means for ecological and environmental protection to participate in the fine management of land and space, and the bottom line of water environment quality is an important grasp for improving the ability of regional water environment management of China. Aiming at the technical problem of defining the bottom line of water environment quality, the technical method of defining the bottom line was refined on the basis of relevant guidelines. Taking Jiangmen City in the Pearl River Delta as the research object, the city was divided into 94 water environment control units. Agricultural sources and domestic sources were the main sources of water pollutants in Jiangmen City. The total amount of water pollutants entering the river in Xinhui District, Kaiping City and Taishan City exceeded 60% of that in Jiangmen City. In order to meet the water quality improvement requirements, the current pollution load of COD, ammonia nitrogen and TP should be reduced by least 14%, 15% and 29% by 2025. A water environment zoning management and control system was established with priority protection areas, key management and control areas and general management and control areas as the main body. The proportion of the three types of zoning areas was 9.4%, 31.9% and 58.7% respectively. This paper discussed the key points of the implementation, renewal and adjustment of the bottom line of water environment quality. The research results can provide theoretical and practical reference for other cities in the Pearl River Delta.