The influx of seasonal migrants has a significant impact on public services in destination places and may reshape the spatial accessibility and equity patterns of park green space (PGS). However, the two-step floating catchment area (2SFCA) method and its extended forms neglect discrepancies between the travel behaviors of seasonal migrants and native residents and thus fail to delineate variations in PGS accessibility and equity in areas with seasonal migrants. To avoid this issue, this study drew on the case of Sanya, a city with large numbers of Houniao, who are primarily retirees leading seasonal migration between the north and the south of China. A multi-group, multi-mode Gaussian-based 2SFCA method was also proposed to evaluate the PGS accessibility and equity before and after the Houniao influx. The method considered the changes in the COVID-19 restrictions from a refined perspective, with fine-scale residential areas being the research units and travel time requested from the web map application programming interface. The results showed that most residential areas were found to have relatively low PGS accessibility and equity levels, except for those in the south-central and southwestern urban areas of Sanya. Both the Houniao influx and lifted COVID-19 restrictions affected the spatial patterns of PGS accessibility and equity. PGS accessibility and equity were decreased by the Houniao influx, whereas walking and public transport within a few residential areas outside Houniao gathering spots improved. This study can serve as a basis for the reasonable planning of PGS and other public services in cities receiving seasonal migrants, such as Sanya.

Subjected to high-intensity development, enclosed tropical islands frequently face severe spatial resource constraints and acute risks of ecological degradation. To seek a comprehensive solution to multidimensional spatial conflicts, this study selected Hainan Island—a region facing development pressure from the Free Trade Port construction—as the study area. By coupling the XGBoost-SHAP, NSGA-II, and PLUS models, we constructed an integrated modular spatial planning framework of “mechanism interpretation—macro optimization—micro reconstruction”. The main findings are as follows. (1) Over the past 15 years, the fragile ecosystem of the island has faced immense pressure. Lacking an external buffer hinterland, land-use transitions exhibited a distinct chain transmission characteristic of “built-up land occupying agricultural land, and agricultural land squeezing forest”. This internal spatial cascading substitution constituted the core mechanism of ESV decline. (2) The reconstruction of the spatial pattern is governed by dual thresholds of natural constraints and economic pulling. A rigid ecological defense baseline (NDVI ≥ 0.40) restricts disorderly urban sprawl, while spatial inflection points (8.46 km from the coastline and 271 m from roads) precisely delineate the coastal and traffic-oriented agglomeration boundaries for built-up land. (3) Among the predefined scenarios, the Sustainable Development (SD) scenario emerges as the optimal pathway. By proactively reducing inefficient agricultural land space at the macro level and strictly implementing identified threshold controls alongside “infill” urban agglomeration at the micro level, the SD scenario is expected to successfully achieve a Pareto-optimal synergy between socioeconomic growth and the restoration of regional ESV. This study not only establishes an objective benchmark for the refined multi-objective spatial governance of the Hainan Free Trade Port, but also provides a referential planning paradigm for vulnerable enclosed islands globally facing high-intensity anthropogenic stress.