[Objective] Water security is an important cornerstone of sustainable development and is closely associated with human well-being. As national parks are typical ecological functional areas, it is important to clarify the complex interaction mechanisms among their various water ecosystem services, which forms the scientific basis for achieving coordinated management of regional water security. [Methods] Based on the InVEST model, PLS-SEM model, Spearman correlation analysis and geographical detector method, this study systematically analyzed the spatiotemporal evolution characteristics and trade-offs/synergies of four key water ecosystem services (water yield, water conservation, soil retention, and water purification) in National Park of Hainan Tropical Rainforest from 2000 to 2022, identified key driving factors, and provided insights for sustainable spatial planning and management. [Results] (1) From 2000 to 2022, the overall status of three water ecosystem services—water yield, soil retention, and water conservation—in National Park of Hainan Tropical Rainforest improved, while the water purification function slightly declined. The water ecosystem services exhibited significant spatial heterogeneity, with the Wuzhi Mountain-Limuling Mountain range showing better water ecological functions than the hilly and plain areas. (2) Except for water yield and soil retention, other water ecosystem services were dominated by synergies with small fluctuations in correlation coefficients, and the synergy between water purification and water conservation was the strongest. (3) Climate change and landscape patterns directly and positively impacted all four water ecosystem services with the strongest explanatory power, while human activities showed a direct negative impact with the weakest explanatory power. [Conclusion] The water ecosystem services in National Park of Hainan Tropical Rainforest generally show an improving trend. This change is significantly influenced by the interaction between climate change and landscape patterns, while the direct impact of human activities is relatively limited. This study proposes a governance framework for water ecosystem services in national parks from four aspects: institutional construction, spatial planning, ecological management, and environmental monitoring. In the future, a theoretical framework for optimizing the water ecological security pattern should be further developed, integrating the needs of regional ecological conservation and spatial governance. This will provide important theoretical support for maintaining the integrity and stability of water ecosystems in national parks and promoting the sustainable governance of territorial space.