Abstract: Phosphorus (P) budget processes and environmental capacity in Culture-based fisheries in large water surface represent a crucial link between nutrient cycling and ecological fishery utilization; however, their quantitative relationships and management response mechanisms remain poorly understood. To reveal the phosphorus budget characteristics of reservoirs and formulate scientific ecological-fishery coupling strategies, we investigated Wangzhai Reservoir. Based on the monitoring of the spatiotemporal variations of physicochemical environmental factors in the water column and P fractions in the sediments, P budget fluxes and environmental capacity were estimated, and two ecologically-coupled fishery strategies were proposed. The results indicate that: (1) Wangzhai Reservoir acted as a net P sink throughout the year, with riverine inputs accounting for 96.1% of total P input. The P retention rate was 17.5%, and the retained P was mainly deposited in the sediments. (2) P exchange at the sediment-water interface exhibited significant seasonal "source-sink" shifts, with sedimentation dominating in summer and autumn, while a transition from sink to source in other seasons. The highest P release flux was observed in winter (0.262 ± 0.055 mg/(m²·d)). (3) The ecological fishery production potential, constrained by environmental capacity, was estimated to be 109 t/a, whereas P removal via fishery harvest accounted for only 7.74% of the annual net P retention. (4) Under different management scenarios, the carrying capacity of low-pollution enclosure aquaculture varied considerably, with estimated potentials of 77.6 t/a and 468 t/a under the “zero-pollution aquaculture” and “water-quality compliance control” scenarios, respectively. This study provides a scientific basis for the green utilization of Culture-based fisheries in large water surface and offers support for the coordinated optimization of water quality regulation and fishery utilization under nutrient load constraints.