Polyethylene glycol (PEG) is a kind of phase change material with high phase change enthalpy and good compatibility with the environment. However, there is relatively large supercooling for PEG, limiting their practical applications. To reduce their supercooling degree, herein we use three different small molecules (acryloyl chloride, acetyl chloride, and thionyl chloride) to modify PEG and study the effects of different end group modification on their phase change properties. Fourier-transform infrared (FTIR) and proton nuclear magnetic resonance (H-1-NMR) spectroscopy are used to confirm the molecular structure of the PEG with different molecular weights and functionalities after chemical modification. Crystal structures of the PEG before and after the modification are verified by X-ray diffractometry (XRD) method and show no change. Differential scanning calorimetry (DSC) results show that the end group modification is quite effective for mitigating the supercooling of PEG with double -OH end groups but not effective for PEG with mono -OH end group. The mechanism for the change of supercooling behavior is proposed. The costs of different modification methods are estimated and compared.