Abstract: Upstream natural ecosystems, protected within national park areas and reservoirs infrastructure can conserve, store and regulate water flow to enhance the availability of water resources and thus meet food, energy and water needs of downstream residents. However, there is lack of quantitative research on the combined contributions of natural ecosystem conservation and reservoir construction to downstream food, energy, and water supply. In this study, the Changjiang River Basin, an important region for the conservation of the Hainan Tropical Rainforest National Park, hydropower development, and food production, was taken as a case study to explore the joint role of national park conservation and reservoir construction in securing downstream food, energy, and water supplies. Besides, the Soil and Water Assessment Tool (SWAT) was leveraged to assess the impact of protected areas on reservoir inflow, and the Water Evaluation and Planning System (WEAP) was used to evaluate the impact of reservoirs on downstream human water demand and subsequently on crop production and hydropower generation. According to the results, the forests protected by national parks could substantially increase reservoir inflows during dry seasons (by 1.34%—22.64%). Reservoirs, in turn, could store water during the rainy season and release it during the dry season to meet the water demands of downstream communities and thus enhance crop production (7.9%). Additionally, national park protection could trigger a 7.3% increase in hydropower generation from reservoirs. As demonstrated by these results, the natural ecosystems protected by national parks and reservoir could jointly ensure the food, energy, and water supply for downstream. In the future, priority should be given to the coordinated management of national parks and their associated infrastructure.