Understanding the intricate relationship between the evolution of territorial spatial patterns and ecosystem services, along with identifying the dominant influencing factors, can provide essential decision-making support for regional territorial development and ecological environmental protection. This study utilized spatial transition matrices and the InVEST model to analyze the spatiotemporal evolution characteristics of Hainan Island's territorial spatial patterns and ecosystem services from 1980 to 2020. Based on this, a comprehensive ecosystem service index (CESI) and a territorial space transformation ecological contribution rate were introduced to assess the ecological effects of territorial spatial pattern evolution. The Random Forest model was then applied to explore the leading factors and mechanisms influencing ecosystem service evolution. The results indicate that: ① Over the past four decades, Hainan Island has seen continuous urban space expansion, a reduction in agricultural space, and an initial increase followed by a decrease in ecological space. The total scale of cross-transformation in territorial space reached 2717.11 km², with the dynamic transformation between ecological and agricultural spaces being the most significant. The conversion of ecological and agricultural spaces into urban space was next in significance, while the transformation of urban space into ecological and agricultural spaces was the weakest. ② The distribution of water yield among ecosystem service functions showed a high in the northeast, low in the southwest pattern, while soil conservation, carbon storage, habitat quality, and the comprehensive ecosystem service index were high in the center, low around the edges. Over 40 years, all types of ecosystem service functions have shown varying degrees of decline, a result of the deterioration trend in Hainan Island's ecosystem service functions outweighing the improvement trend. The conversion of ecological space to agricultural and urban spaces was a significant reason for the regional ecological service function deterioration. ③ The functionality of Hainan Island's ecosystem services was influenced by a combination of natural geography, transportation location, socio-economic factors, and policies. Transportation location, socio-economic, and natural geographic factors had significant impact, while policy factors were relatively weak. Furthermore, the relationship between these influencing factors and ecosystem service functions was nonlinear.