Given the importance of protected areas (PAs) in promoting the balance between conservation and sustainable development, it is crucial for planners and decision-makers to focus attention on the core areas that are of priority to PAs. However, few studies have addressed the balance between ecosystem service provision in PAs and regional development demands based on spatial valuation data. Herein, we present an integrated approach using the Cangshan Nature Reserve as a case study with the aim of identifying the core conservation areas of the reserve—the only national nature reserve in the 18-creek watershed of Cangshan—under different urbanization scenarios. The results show that the overall farmland in the watershed decreased and the architectural area increased, but the land use in the nature reserve remained stable from 1995 to 2035. With the increase in demand for water in the watershed, at least 24.3% of the Cangshan Nature Reserve should be designated as core conservation area to ensure the maintenance of sufficient water quantity and quality. This study can be used as a reference for the sustainable management of PAs based on our example of balancing ecosystem service provision and demand in a single watershed.