The thermosetting solid-solid phase change materials (SSPCMs) composed of poly(ethylene glycol)-based two components, isocyanate-terminated prepolymer and tetrahydroxy prepolymer, were synthesized via solvent-free bulk polymerization, endowing the thermosetting PCMs with flexible application capability. The chemical structures, crystalline properties, phase change properties, thermal energy storage mechanism, thermal reliability and stability of the synthesized PCMs were respectively characterized by Fourier transform infrared spectroscopy, X-ray diffraction, differential scanning calorimetry, polarizing optical microscopy, accelerated thermal cycling testing and thermogravimetric analysis. The intrinsic crystalline structure in SSPCMs is not influenced by the crosslinking reaction between two components. However, compared with the poly(ethylene glycol) (PEG), the lamellae and spherulite size in SSPCMs decrease to some entente and dramatically, respectively. The SSPCMs have proper phase change temperature of about 45 °C and high phase change enthalpy of about 100 J/g much less than the PEG, resulted from the small spherulite and lamellae size working together. The SSPCM-based thermal energy storage relies on the phase change from the crystalline state to the amorphous state and vice versa. The SSPCMs have good thermal reliability and stability with the degradation temperature higher than 300°C.