Abstract: Developing offshore aquaculture is crucial for improving the offshore environment, ensuring national food security, and promoting the sustainable development of the marine fishery economy. With the growing development of the offshore aquaculture industry in China, how to select and delineate suitable aquaculture areas at the initial stage of offshore cultivation, and thus to effectively provide guidance and references for subsequent site selection and aquaculture management has become an urgent issue to be addressed. By summarizing relevant literature and integrating the characteristics of different offshore aquaculture equipment, this study firstly established an indicator system for aquaculture area selection based on the conditions of the environment and natural resources. The indicator system consists of three major categories: natural environment and resource conditions, feasibility of selection and delineation, and the maximum number of aquaculture facilities to be deployed, and comprises seven specific indicators: geographical locations and related supporting facilities, water depth, seabed sediment types, marine hydrological and ecological environment and fishery resources, the coordination of surrounding exploitation activities, the conformity of related policies and plannings, and the aquaculture facilities capacity. Meanwhile, considering the characteristics of offshore aquaculture area selection, the overlapping map method was proposed to conduct this selection process. On this basis, the established index system and method were applied to the offshore aquaculture zone selection outside Xiamen Bay, Fujian Province. In this process, the aquaculture zones that met the requirements of suitability were selected, and feasibility analyses were conducted, and aquaculture facilities capacity in each selected zone was proposed. The results indicated that there are three suitable zones for offshore aquaculture outside Xiamen Bay waters (Zone A, Zone B, and Zone C), with areas of 18.27 km², 34.27 km², and 4.40 km², respectively. The maximum number of truss-type platforms that can be deployed in each of the above-mentioned zones is 108, 203, and 26, respectively, and the maximum number of aquaculture vessels that can be deployed is 50, 111, and 14, respectively. This study aims to provide insight and empirical reference to related research in other coastal areas of China.