The intermittent nature of solar irradiation poses significant challenges to the energy transfer stability of conventional solar-thermal conversion materials. To address this, solar-thermal phase change materials (PCMs) integrate thermal conversion and storage, mitigating temporal and spatial discontinuities of solar energy. However, existing PCMs suffer from limitations such as inadequate solar absorption, leakage, and low phase change enthalpy. In this study, a novel binary eutectic PCM comprising 1,10-decanediol (DDL) and paraffin wax (PW) was synthesized via melt blending. Expanded graphite (EG) was incorporated as a thermal conductivity enhancer and structural support, with a shape-stabilized composite PCM fabricated through vacuum impregnation. The DDL-PW/EG composite achieved a high encapsulation efficiency of 90%, exhibiting a phase change temperature of 63.2 °C and a latent heat capacity of 203.9 kJ·kg−1. Notably, its thermal conductivity reached 6.89 W·m−1·K−1, 13.78 times higher than that of pristine DDL-PW, while maintaining 85.0% photothermal conversion efficiency. Accelerated thermal cycling tests (200 cycles) revealed negligible degradation in phase change temperature and enthalpy, underscoring exceptional thermal reliability of the composite. Combining robust thermal storage performance with efficient solar-thermal conversion, the DDL-PW/EG composite demonstrates significant potential for advancing solar energy harvesting and storage applications.