Abstract: The use of phase change materials (PCMs) with high energy storage density is an ideal method to solve the problem of uneven and discontinuous of solar energy. At present, the common phase change media are mainly low-temperature solid–liquid phase change materials (SL-PCMs), which have shortcomings such as easy leakage, low thermal conductivity (TC), and low photothermal conversion performance. In response to these problems, a research idea of using pentaerythritol (PE) as a solid–solid phase change heat storage medium and gallic acid-modified graphene nanosheets (GNPs) as light absorbers and thermally conductive filler was proposed in this paper. The results showed that the composite phase change materials (CPCMs) containing 15 wt% GNPs had the highest photothermal conversion efficiency, reaching 93.86%, which was 247.76% higher than that of PE, and the latent heat of phase change was 190.88 J/g. Compared with PE (TC is 1.02 ÂW/(mÂ·K)), the TC of 15 wt% GNPs-CPCMs reached 3.86 ÂW/(mÂ·K), which increased by 278.43%. At the same time, their phase-change temperature remained at a medium–high temperature around 185 °C, and the UV–Vis spectra showed that the CPCMs also had high absorbance under visible light. This suggested that solid–solid composite phase change materials (SS-CPCMs) with high absorbance and TC provided a new approach for solar photothermal conversion and storage.