Polyethylene glycol (PEG) is extremely challenging for solar-thermal energy conversion and thermal management due to its solid-phase rigidity, liquid-phase leakage, low thermal conductivity and weak light absorption. In this paper, a solid-solid phase change material (HDPCM) is obtained by the block copolymerization method with an enthalpy of melting of 102.3 J/g, demonstrating a high thermal energy storage capacity. We have innovatively combined this HDPCM with hydrothermal carbon (HTC) to obtain a composite PCM with excellent thermal properties and solar thermal energy conversion. HDPCM-HTC-3 has significant mechanical flexibility shape cutability, and foldability. The photothermal conversion efficiency is evaluated at 83.7%, indicating its superior ability to convert solar energy into thermal energy. The initial pyrolysis temperature exceeds 200 °C, indicating good thermal stability. After 101 heating/cooling cycles, the chemical and thermal properties remain essentially unchanged. Simulated thermal management experiments show that this HDPCM-HTC-3 has good thermal management performance, and that hot water from a loaded HDPCM-HTC-3 conical flask can have significant thermal insulation performance, with the temperature being constant for approximately 320 s over a range of approximately 45 °C. The HDPCM-HTC-3 exhibits significant temperature-controlled thermal management effects, showing potential application prospects.