A series of form-stable polyethylene glycol/activated carbon (AC) composites were prepared via a vacuum-assisted infiltration method, where polyethylene glycol (PEG) was used as an organic phase change material (PCM) and AC was used as an inorganic supporting matrix to prevent the leakage of the PCM during phase change period. The chemical structural, thermal properties, thermal stability, and reliability of PEG/AC composite PCMs (CPCMs) with various mass loadings of PEG2000 was investigated by N2 adsorption analyzer, Fourier transformation infrared (FT–IR) spectrometer, differential scanning calorimetry (DSC) and thermogravimetric analyzer (TGA), respectively. The form stability and thermal energy storage and release properties of PEG/AC CPCMs were also experimentally studied. The results showed that PEG/AC CPCM with 80 mass% PEG2000 loading displayed reasonable latent heat, suitable phase transition temperature, good thermal reliability, enhanced thermal conductivity, and thermal energy storage and release performances, as well as excellent form stability, which are suitable for the practical applications of PCMs for thermal energy storage.