Against the backdrop of the intensification of global climate change and the normalization of extreme weather events in cities, the traditional blue line control system is facing an urgent need for systematic reconstruction. The logic of delineating the blue line dominated by engineering thinking reveals four dilemmas in dealing with compound flood risks, coordinating urban spatial development, and other aspects. Firstly, the scope of the blue line definition lags behind the concept of ecological water management. The current delineation of the blue line still adheres to the cognitive framework of traditional hydraulic engineering, focusing on the spatial areas of obvious water bodies such as rivers and reservoirs. There is a lack of legal status recognition for distributed storage and regulation facilities such as rainwater storage and regulation ponds and ecological ponds that have emerged in the construction of sponge cities. Secondly, the engineering-oriented approach to delineating the estuary line has systematic defects. The method of delineating the estuary line dominated by a single flood control standard is prone to fall into the path dependence of “surrounding the city with dikes” when dealing with extreme rainfall events. Thirdly, the control lines on the land area are rigid and disrupt the organic spatial connections. The current land area control mode of “equidistant setback” ignores the dynamic coupling relationship between the natural morphological characteristics of rivers and the urban spatial structure. Fourthly, the waterfront space control system shows multi-dimensional disconnections. The lack of specific control at the land use level leads to the failure of the transmission between the guidance of the spatial form and the control of land development. Based on this, this paper proposes corresponding planning ideas. Firstly, establish a dynamically evolving connotation system of the blue line. Facing the long-term impacts of climate change and the dynamic evolution of the urban water system, on the basis of protecting traditional rigid water bodies such as rivers and lakes, incorporate distributed storage and regulation facilities such as rainwater storage and regulation ponds, ecological ponds, and artificial wetlands into the legal control scope of the blue line, and establish a composite blue line system covering “gray-green” hybrid infrastructure. Secondly, innovate the delineation of the estuary line with upstream and downstream flood control coordination. At the overall level, comprehensively consider the flood control strategies of the upstream and downstream areas of the river basin, give full play to the flood storage and detention functions of various storage and regulation water bodies, relieve the flood control pressure within the city, take into account the value balance of “safety-ecology-development”, and reasonably delineate the estuary line. Thirdly, implement differentiated land area control strategies. Comprehensively consider the important ecological spaces and urban spaces along the water system, with appropriate flexibility, and achieve the precise adaptation between the physical geographical characteristics and the urban functional zoning. In addition, according to the different characteristics of different control objects, adopt a control method that combines rigidity and flexibility. Fourthly, improve the planning transmission mechanism that combines guidance and control. Transform the key control elements in the waterfront space design guidelines into clear indicators and requirements, and use them as the basis for compiling the detailed plans of the areas along the line, so as to achieve the transmission from spatial design to land management.Taking HefeiCity as an empirical example, this paper introduces the specific paths and methods for delineating the blue line. Firstly, expand the objects of blue line delineation, coordinate various storage and regulation facilities, and incorporate some seasonal, underground, and small-scale storage and regulation facilities into the objects of blue line delineation. Secondly, coordinate various water-related plans, formulate a systematic stormwater management plan, and coordinate the urban flood control plan and drainage plan. On the basis of systematically integrating the flood control and drainage measures of the upstream and downstream areas, comprehensively delineate the urban blue line. Considering the terrain conditions where the northwest of Hefei urban area is high and the southeast is low, as well as the existing foundation of water conservancy facilities, it is recommended to adopt the flood control strategy of “blocking upstream, releasing downstream, and diverting on the sides”. By improving the flood storage capacity of the upstream and downstream areas, the flood discharge pressure within the city can be relieved. Thirdly, combine relaxation and tightening, integrate rigidity and flexibility, and explore refined methods for blue line delineation and control. Combining relaxation and tightening means adopting the mode of “benchmark width + local correction”, that is, setting a certain benchmark width by comprehensively considering various requirements such as river flood control safety, ecological conservation, and recreational activities, and making local corrections in combination with the specific situations along the line. Integrating rigidity and flexibility means comprehensively using various control methods such as line positions, point positions, and indicators. Fourthly, formulate waterfront space control guidelines that shift from “separation of the city and water” to “coupling of the city and water”, and propose an indicator system with 10 items at three levels: ecological green, landscape features, and openness and sharing.