In the context of global climate change, the sensitivity of ecosystem services (ESs) and their relationships to aridity are increasing, and the areas affected by aridity continue to expand. Previous research has largely focused on ES changes in arid regions under intensified aridity conditions and has overlooked humid and semi-humid regions, which are more sensitive to water loss. In this study, we applied a generalized additive model (GAM) to analyze the nonlinear responses of ES trade-offs to aridity in humid and semi-humid regions. The results indicated that at an aridity index (AI) of approximately 0.26, the trade-off intensity of two ES pairs-carbon sequestration vs. water yield and habitat quality vs. water yield-reached a peak. Additionally, structural equation modeling (SEM) based on threshold partitioning revealed significant shifts in ES trade-off drivers below and above the AI threshold. In areas with AI < 0.26, worsening meteorological and soil conditions exacerbated ES trade-offs, whereas in areas with AI > 0.26, competition for water resources between human activities and vegetation became the primary driver of intensified trade-offs. These findings highlight the need for targeted management strategies to maintain ecosystem stability in humid and semi-humid regions and provide valuable guidance for responding to increasing aridity risks.