The Hainan Eld's deer (Rucervus eldii hainanus) is a nationally protected Class I species in China and one of the most endangered cervids globally. Climate change is profoundly affecting its population dynamics by driving habitat loss and fragmentation and altering the extent of suitable habitat. To assess habitat suitability, we employed an optimized MaxEnt framework integrating 154 species occurrence points and 14 environmental predictors, which uniquely included extreme climatic indicators for heat-induced drought and torrential precipitation. Suitability maps were generated for the present-day baseline, contemporary extreme events, and three future scenarios (SSP126, SSP245, SSP585) for the 2050s and 2070s. Six bioclimatic factors emerged as primary drivers shaping the deer's distribution: precipitation during the driest month (Bio14), mean temperature of the driest quarter (Bio9), isothermality (Bio3), precipitation in the warmest quarter (Bio18), mean diurnal temperature variation (Bio2), and precipitation in the wettest quarter (Bio16), together accounting for 73.4% of the permutation importance. Under current baseline climate, potential habitat (totaling 8501.60 km2) predominantly occurs within open savanna landscapes of southwestern Hainan, whereas highly suitable zones cover merely 956.47 km2, equivalent to 2.82% of the island's total area. Relative to baseline estimates, suitable areas expanded substantially under drought extremes but diminished under anomalously wet conditions. Future projections reveal habitat expansion across all climate trajectories, with the most pronounced gain under SSP126 in the 2050s, expanding total suitable habitat to 12,746.68 km2. Concurrently, the centroid of suitable habitat shifts southwestward. Collectively, these findings suggest that across diverse climatic regimes, the Hainan Eld's deer consistently favors relatively warm and arid environments. This work offers a scientific foundation for climate-resilient conservation strategies, habitat rehabilitation, and population stewardship of island-endangered ungulates.