The ecosystem service functions of wetlands served as the key foundation for realizing the important value of international wetland cities. However, existing studies on ecological assessment of wetlands before and after the establishment of international wetland cities mostly focused on the changes in wetland types and dynamics of wetland areas, with relatively less attention paid to the spatiotemporal evolution of their ecosystem service functions and policy-driven factors. To address this, this study adopted the "type-function-policy" comprehensive framework. Based on refined classification of urban wetlands (10-meter spatial resolution, 13 wetland types), we first identified and quantified changes in wetland types and areas and then employed the InVEST model to evaluate changes in wetland ecosystem service functions before and after Haikou's certification as an international wetland city (2016 vs 2022). In addition, we explored changes in wetland types within the areas that experienced significant changes and analyzed the impact of wetland protection and restoration policies. Through this integrated approach, this study is intended to provide scientific evidence and practical support for the formulation of more targeted and effective wetland protection strategies and management measures in international wetland cities. The results indicated that: (1) Before and after certification, the total wetland area remained relatively stable, but marshes increased significantly, approximately 12.6%, while aquaculture ponds decreased sharply, approximately 36.67%. (2) Haikou's wetland ecosystem service functions showed significant improvements in wetland carbon storage (increasing by approximately 5.13%) and water purification functions (reducing nitrogen and phosphorus by approximately 3% and 3.55% respectively), but declined slightly in flood regulation (reducing by approximately 1.47%) and water conservation functions (reducing by approximately 2.12%). (3) Spatial variations in ecosystem services were predominantly stable. Carbon storage and water conservation changes were most prominent. The areas of the variation zones were 57.7 km² and 50.32 km² respectively. Within these change zones, the changes in wetland types were mainly reflected in the conversion from aquaculture ponds to coastal marshes and the mutual conversion between forest and marshes. The northern part of Meilan District and the southern part of Qiongshan District were typical areas exhibiting pronounced shifts in wetland ecosystem service functions. In view of this, the protection and management of urban wetland ecosystem in the future should not only focus on monitoring and regulating changes in wetland areas and types, but also attach greater importance to tracking the spatiotemporal dynamics of their ecosystem service functions, so as to maximize the exploration of wetland values.